This essay analyzes what it means for China if the United States and Russia fail to replace or renew the New START. For the first time in many decades, China would no longer enjoy the security benefits of the existing U.S.-Russia bilateral nuclear arms control regime. If the New START transparency and verification measures cease to exist, or worse, if the United States and/or Russia start to reverse their nuclear reductions, what would be the impact on China’s strategic nuclear deterrent and China’s nuclear thinking? How would China respond? These are important issues for U.S. and Russian leaders to consider, before they make changes to the existing bilateral nuclear arms control institutions.

Tong Zhao
Tong Zhao is a senior fellow in Carnegie’s Nuclear Policy Program.
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Impact of Numerical Growth of U.S./Russian Strategic Nuclear Weapons on China’s Strategic Nuclear Deterrent

With the possible demise of the U.S.-Russia bilateral nuclear arms control regime, China’s most immediate concern would be how this might affect China’s strategic nuclear deterrent. For decades, the concern about the credibility of its second-strike capabilities has been the primary driving force of China’s nuclear modernization efforts. Today, after significant investments into building and fielding more survivable nuclear forces, there is still a considerable uncertainty over how confident Chinese decision-makers should feel about China’s capacity to execute an effective retaliatory strike after absorbing a disarming first strike and bring about an unacceptable damage to the United States. In the public domain, some Chinese scholars evaluate China’s second-strike capability to be far from being 100-percent assured.1  Such a view that China needs to further strengthen its strategic nuclear deterrent capabilities is widely shared within the Chinese nuclear expert community. In a post-New START world of unconstrained U.S. and Russian forces, the numerical increase in the U.S. and Russian strategic nuclear weapon arsenals could exacerbate China’s concern about the credibility of its nuclear second-strike capabilities.

Near-Term Impact

Some U.S. scholars have pointed out that existing U.S. ICBMs and SLBMs have become much more capable of executing counterforce strikes against certain elements of Chinese land-based missiles and other strategic nuclear targets.2  In the case of the U.S. SLBMs, for example, the deployment of the new MC4700 burst-height compensating super-fuze on the W76-1/Mk4A warhead since 2009 has significantly increased the accuracy and then the hard target kill capability of such missiles.3 Looking into the future, without the constraint of the New START, by the late 2020s the United States could deploy 50 more ICBMs, 400 more ICBM warheads, 48 more SLBMs, and 216 more SLBM warheads.4  For China, this would represent a considerable additional counterforce strike potential of the United States. Moreover, some U.S. scholars conclude that U.S. strategic bombers that carry nuclear gravity bombs and air launched cruise missiles are also highly lethal counterforce weapons against some Chinese nuclear forces. In a 2007 article, Keir A. Lieber and Daryl G. Press calculated that three B-2 bombers, each armed with 16 variable-yield nuclear gravity bombs, would be enough to destroy 20 DF-5 silo-based ICBMs—the backbone of China’s strategic deterrent forces before road-mobile ICBMs became fully operational.5  By the late 2020s, the United States could increase the number of its nuclear-capable strategic bombers from 60 to 90 if it reversed the New START bomber conversions to conventional-only capacity. From the Chinese perspective, this would be another important increase of the U.S. counterforce capabilities.

With that said, the impact of the numerical growth of the U.S. nuclear forces on China’s future nuclear modernization is uncertain. One important reason is that China perceives the United States as already possessing a formidable nuclear counterforce capability today. With the current U.S. nuclear arsenal, if the United States wanted to pursue a disarming first strike against China, it probably would not need to employ all its nuclear weapons. In other words, more of the same weapons would not significantly increase the chances of a successful U.S. disarming strike on China, whereas a qualitative improvement in some aspects of the U.S. capabilities could have a much greater impact on the U.S. counterforce potential against China, such as a more advanced intelligence, surveillance, and reconnaissance (ISR) capability to identify China’s hidden ICBM silos or to track China’s road-mobile ICBM vehicles. Therefore, the credibility of China’s second strike is not very sensitive to a relatively moderate numerical growth of U.S. strategic nuclear weapons. 

In all likelihood, according to a 2019 CNA analysis, the potential growth of U.S. strategic nuclear weapons by late 2020s would be moderate: the numerical growth of U.S. strategic delivery vehicles would be 18.3 percent or less, compared with the scenario in which the United States exercises self-restraint and keeps its nuclear capabilities at the New START level; Similarly, the projected numerical growth of U.S. warheads would be 22.0 percent or less.6  Such a relatively moderate increase in the near-term future would not make U.S. counterforce strike potential against China much greater than the current situation.

Long-Term Impact

For the long-term future scenario that extends far beyond the late 2020s, the United States could significantly increase its strategic nuclear stockpile beyond the current level. It is probable that Chinese leaders would view a much larger U.S. strategic nuclear arsenal as representing a much greater threat to China that would deserve forceful countermeasures. But even under those circumstances, China would still have options to enhance its nuclear deterrent capabilities in areas that the growth of U.S. strategic nuclear weapons would not significantly affect.

For example, the numerical growth of the U.S. strategic nuclear weapons would not seriously affect the survivability of China’s strategic nuclear submarines (SSBNs), once they are at sea. Therefore, China would have the option of raising the relative importance of SSBNs within its nuclear triad by allocating more resources from other programs to its SSBN modernization program. The objectives would be to increase the quietness and reliability of its SSBN fleet and to adopt an operational strategy that reduces the time of SSBNs staying in port (when the SSBNs are more vulnerable).

More generally, the vulnerability of China’s nuclear forces is and probably will continue to be more dependent on the capabilities of the U.S. ISR systems than the quantitative or qualitative evolution of the U.S. nuclear forces. If the United States can locate and keep track of all Chinese nuclear weapons with high confidence, then the United States should have more than enough options to strike and destroy them. China’s efforts in recent decades to focus on developing mobile nuclear weapons delivery systems can make it more difficult for the United States to ensure the effectiveness of its ISR systems. The ISR challenge may continue to impose the greatest constraint on the U.S. counterforce capabilities against China.

Furthermore, to reduce their vulnerabilities, China could choose to put its nuclear weapons on higher alert during peacetime (by means such as keeping the warheads mated to the missiles and keeping the nuclear-armed missiles dispersed) and even shift its nuclear posture from retaliation after absorbing a first strike to launch under attack (LUA). Although China’s silo-based liquid-fueled DF-5 ICBMs may be technically difficult to maintain a high alert status during peacetime, China could take measures to put its road-mobile ICBMs on higher alert. After China’s new DF-41 road-mobile ICBMs become fully operational, these missiles, together with the existing DF-31As (and their variants) would constitute the backbone of China’s land-based ICBM forces. Additionally, China is on path to fielding a new long-range strategic bomber and could choose to put such bombers armed with nuclear gravity bombs and/or cruise missiles on high alert during peacetime. All of these measures would require China to possess a capable strategic early warning system, which China is reportedly already developing. The Science of Military Strategy, a textbook authored by some Chinese military academics and published by the PLA Academy of Military Science in 2013 also considers the LUA posture as a potentially useful measure to ensure China’s nuclear deterrent.7  Shifting to a LUA posture and relying on its untested early warning system for making decisions on nuclear use would introduce unprecedented risks of false warning. From the perspective of crisis stability, this would be a more serious consequence than for China to build up its SSBN forces.

Transparency and Possible Chinese Interpretation of U.S./Russian Numerical Growth 

In case of the demise of the New START in the future, a lack of transparency and verification measures could have two effects. First, it could increase Chinese uncertainty about the sizes of U.S. and Russian strategic nuclear arsenals and so exacerbate China’s concerns about their numerical growth. Second, a lack of transparency would lead the U.S. and Russian arsenals to grow faster than they otherwise would, and lead China to attribute such growth to more aggressive intentions of the two big nuclear powers. 

Since the establishment of formal nuclear arms control mechanisms between the two leading nuclear powers, China seems to have been reassured by the transparency and verification measures built into the U.S.-Russia bilateral arms control agreements. Although China cannot officially and independently verify the implementation of U.S. and Russian nuclear reductions, Chinese experts rarely challenge the numerical figures of the U.S. and Russian nuclear arsenals, as claimed and verified by U.S.-Russia bilateral mechanisms. Whether that confidence would remain, after the U.S.-Russian bilateral transparency and verification measures end, is highly uncertain.

It is a question how China’s decision-makers would evaluate the degree of possible U.S. and Russian nuclear growth in the unconstrained scenario. It is likely that Chinese technical experts who conduct in-depth research and who closely follow the U.S. and Russian nuclear development and arms control agreements would know that even in the worst-case scenario the growth of the U.S. and Russian strategic nuclear weapons by the late 2020s would be moderate, as their growth would have been limited by their lack of technical and financial capabilities to mass produce new warheads and delivery vehicles. But whether such an understanding would become the mainstream understanding within the Chinese nuclear policy community is uncertain. Chinese policy experts, especially those who do not have in-depth knowledge on technical issues, might overestimate how much the U.S. and Russian arsenals would grow. In the recent history, there are ample examples in which Chinese policy experts embraced much higher-level threat perceptions toward certain U.S. military development and deployment, than many Chinese technical experts did.8  In those cases, the perception gap between mainstream Chinese policy experts and mainstream U.S. experts was significant. When it comes to China’s decision-making, the influence of the Chinese policy experts can be very important.

As the U.S.-China relationship becomes increasingly competitive and even hostile, the chances for Chinese policy experts and decision-makers to overestimate the aggressiveness of the U.S. and Russian intentions would be considerable. As pointed out by the CNA report, the end of transparency and verification measures could generate mutual worst-case thinking and planning in the United States and Russia on strategic nuclear weapons. Watching an intensified U.S-Russian nuclear competition from the side, it is likely that China would not see this as a result of unfortunate U.S.-Russian security dilemma generated by mutual worst-case thinking. Instead, China would probably conclude that both the United States and Russia are driven by aggressive goals to expand their nuclear capabilities. As a result of China’s deep suspicion toward the United States, China might especially attribute the new nuclear growth to a renewed U.S. ambition to seek nuclear primacy (first-strike capability). Even worse, China might see itself, rather than Russia, as the real target of such a U.S. nuclear growth. For decades, China has had concerns that the United States is interested in obtaining nuclear primacy over China; it has also believed that the United States is much closer to achieving nuclear primacy over itself than over Russia and therefore may be particularly motivated to aim its nuclear growth at China as it sees China as its most important long-term competitor. If China concludes that the U.S. nuclear growth is a result of the U.S. pursuing nuclear primacy against China, Beijing could make even greater efforts on its own nuclear modernization.

Impact of Non-Numerical Factors on China’s Nuclear Modernization

Non-Numerical Factors That Affect China’s Second-Strike Capability

In addition to the numerical growth of the U.S. and Russian strategic nuclear capabilities, other qualitative and structural changes in the U.S. and Russian strategic nuclear systems could have an equal or even greater impact on China’s perception of its second-strike capability. And due to China’s much greater concern against the United States than against Russia, it would be particularly sensitive to any such U.S. changes.

By the late 2020s, the numerical growth of the U.S. and Russian strategic nuclear weapons would be moderate, but the two countries could take measures to change how such weapons are deployed, which could increase China’s threat perception. For example, in the unconstrained scenario, if the United States reconverted 30 B-52H strategic bombers for nuclear missions that it previously converted to conventional-only status, it would have the option of deploying more of such nuclear-capable bombers to places near China such as Guam. Such deployment could exacerbate China’s threat perception of the U.S. preemptive use of nuclear weapons, as the relatively short distance from China from such places would leave Chinese decision-makers less time to respond. Nuclear-capable bombers deployed near China in the Asia Pacific region could be viewed by Beijing as a greater counterforce threat than U.S. ICBMs deployed in its homeland. 

Similarly, today the United States has 14 Ohio-class SSBNs. Among them, eight are assessed by Hans Kristensen and Robert Norris to operate in the Pacific Ocean from their base near Bangor, Washington, and six operate in the Atlantic Ocean from their base at Kings Bay, Georgia.9  In the unconstrained scenario post-New START, the United States in theory would have the option of reactivating four SLBM launch tubes on each of the SSBN operating in the Pacific Ocean, and keeping deactivated four launch tubes on each of the SSBN operating in the Atlantic Ocean unchanged. Such a policy would bring a total of 32 additional deployed SLBMs that could be used to target China, whereas no additional deployed SLBMs would target Russia. This would demonstrate a further shift of strategic military focus from Russia toward China and could cause far greater threat perception in China than the moderate growth of the number of weapons. In practice, the chances for the United States to adopt such a policy are low. If the United States wanted to reverse the New START reductions, it probably would want the deactivated SLBM tubes on all of its SSBNs (regardless of where they are deployed) to be available. However, the point of this hypothetical example is that internal adjustment/re-allocation of deployed capabilities in the future, including their geographical deployment areas, could greatly affect China’s threat perception, in addition to the numerical growth of the overall U.S. arsenal.

Furthermore, what type of nuclear weapons the United States would deploy on its strategic systems by the late 2020s also matters more than the number of weapons to be deployed. If the United States continues to focus on weapons of improved accuracy, flexible yields, greater rapid missile retargeting capability, and less radioactive fallout and thus less collateral damage to civilians, as some U.S. scholars believe has been the recent trend of U.S. weapon development,10  China’s concern about the United States shifting toward acquiring more effective counterforce capabilities and thus leaning more heavily on counterforce strategies would grow.

Existing U.S.-Russia nuclear arms control agreements have not captured such issues related to specific deployment strategy and qualitative improvement of strategic nuclear weapons. At the end of the day, however, these issues play the most important role in shaping China’s interpretation of why the United States (and Russia) in the future might choose to increase its strategic nuclear arsenal. If China believes the United States would shift toward seeking nuclear primacy over China, through building up its counterforce capabilities and adopting nuclear warfighting postures, it could take radical measures to enhance its second-strike capability even if the numerical growth of U.S. strategic nuclear weapons would be moderate in the near-term future.

Perception of Prestige and Diplomatic Leverage

China’s concern about the credibility of its second-strike capability has been the primary driving force of its nuclear modernization efforts. However, other non-military factors seem to increasingly affect China’s thinking about the need to modernize its nuclear forces. The possible demise of U.S.-Russian arms control agreements could further raise the importance of such non-military factors.

For example, some Chinese analysts and commentators have started to adopt the view that nuclear capabilities can translate into international prestige and diplomatic leverage. They believe a bigger nuclear arsenal is necessary to convince the other major powers—and especially the United States—of China’s military strength and thus would make them really listen to China’s views on important foreign policy issues and would ultimately make them treat China equally and fairly. They attribute the U.S. “strategic arrogance” toward China mainly to its “absolute nuclear superiority” over China and believe that if China had had a bigger nuclear arsenal, the United States would not have interfered in the South China Sea and over the Taiwan Strait. Therefore, they worry that continued U.S. “nuclear superiority” would keep fueling dangerous U.S. “military provocations” toward China.11  This view that a bigger nuclear arsenal would win China greater international respect and bigger diplomatic leverage did not seem to be embraced by China’s first- and second-generation paramount leaders such as Mao Zedong, Zhou Enlai, and Deng Xiaoping.12 However, whether China’s current paramount leader rejects such a view is unknown.

The Nuclear Posture Review report of the Trump administration states that the U.S. nuclear deterrent “ensur[es] that our diplomats continue to speak from a position of strength on matters of war and peace.”13  This implicit endorsement of the belief that nuclear weapons translate into diplomatic leverage, together with President Trump’s doctrine of “peace through strength,” helps reinforce the view within certain quarters of the Chinese expert community that it is a universal maxim, endorsed by others including the United States, that a bigger nuclear arsenal delivers greater political benefits. Under such circumstances, if the United States and Russia start to build up their nuclear forces after the demise of the New START, the nationalist voices in China would be further strengthened that the size of the nuclear arsenal does matter and that China needs to expand its nuclear capabilities too, in order to compensate for the growth of the U.S. arsenal and to build a Chinese nuclear force that is “commensurate to” China’s international status in the world.14  At the minimum, the reversal of the U.S. and Russian nuclear reductions could motivate China to consider raising the role of nuclear weapons in its national security strategy, which might lead to even higher-level investments into its nuclear modernization programs than over the past years.

Resource Availability and Arms Competition

Since its first nuclear explosion in 1964, China has so far kept its nuclear arsenal at a relatively small size and avoided building a large arsenal. This was partly due to the belief of China’s first- and second-generation paramount leaders that a small nuclear arsenal would be enough for the purpose of strategic deterrence. At the same time, the lack of sufficient economic and technological resources might have also contributed to China’s small nuclear arsenal. As a result, even though China’s second-strike capability has never been fully “assured,” China had to prioritize and allocated resources to those weapon systems that could most effectively enhance the survivability and credibility of its strategic deterrent. In recent years, however, with the rapid economic development, the resources constraint on China’s nuclear programs gradually disappeared. Accordingly, there have been signs that some of China’s nuclear modernization programs seem to be gradually driven by resource availability.

China’s development of new nuclear-capable systems that are not particularly reliable and survivable if not kept on high alert includes systems such as silo-based liquid-fueled ICBMs with multiple independently targetable reentry vehicle, a new long-range strategic bomber, and a reported new air-launched ballistic missile.15 As the military gets access to increasingly available resources, parochial and bureaucratic interests may have a growing influence over China’s nuclear modernization decision-making. As the Rocket Force manages China’s land-based nuclear missiles, the Navy runs the nuclear strategic submarines, the Air Force operates the strategic bomber fleet, and the Strategic Support Force contributes to the command, control, and communication capabilities, all these services have inherent interests to protect and grow their nuclear profile. Against this background, U.S. and Russian decisions to increase their strategic nuclear weapons would play more effectively into those voices of parochial and bureaucratic interests within China.

At this moment, most Chinese experts are very confident about China’s economic potential to sustain high-level investments into China’s overall military modernization programs for the foreseeable future. With that said, China’s economic growth is slowing down and its economic system has accumulated chronic structural problems over the past period of rapid growth. With the additional pressure from the ongoing U.S.-China trade war, the sustainability of China’s high economic growth rate in both the near- and long-term future looks increasingly uncertain. In case of a major economic downturn, the existing resources available to the military would be undercut. If this happened, the future sustainability of China’s nuclear modernization programs could be put into questions. The degree of that impact, however, is very hard to predict.

Asymmetric Trilateral Nuclear Arms Competition and China’s Participation in Arms Control

According to the above analysis, in a post-New START world, if the United States and Russia start to build up their strategic nuclear arsenals, China would very likely follow suit and redouble efforts to modernize its nuclear weapons. With regard to the geopolitical relationships among the three countries, although Russia and China share a common concern against the United States and have engaged in increasingly closer cooperation in some strategic security areas such as missile defense, Russia and China have not built deep enough strategic trust to enable them to join forces in countering the perceived U.S. nuclear threat. In the nuclear domain, Russia and China would probably remain independent players in planning and developing their nuclear forces and they would likely continue to maintain some deterrent capabilities and postures against each other as well. To some extent, Russia and China are in a competition to make each other the main target of the U.S. strategic concern and therefore to relieve themselves from the burden and pressure of engaging in an all-out strategic competition with the United States. For such reasons, a post-New START world would likely witness an asymmetric trilateral nuclear competition in which all players seek to deter and hedge against each other and the U.S.-Russian and U.S.-Chinese competitions are particularly intense. Such a complex trilateral relationship can be highly volatile, as everyone responds to the change of capability and/or posture of everyone else.

In practice, the trilateral competition would be even more intense and complex than discussed above, because non-strategic nuclear weapons and non-nuclear strategic weapons could play an even greater role in creating threat perceptions and in contributing to security dilemmas among the three countries. For instance, both Russia and China see U.S. missile defense as posing the most serious threat to their nuclear second strike capabilities. Their concerns about missile defense appear to have been the greatest incentive behind their development of new nuclear capabilities. All three countries have also clearly entered a negative action-reaction cycle in a competition to develop new strategic weapon systems such as hypersonic boost-gliders. Whether conventionally-armed hypersonic weapons will be massively deployed and thus threaten nuclear weapon systems and whether hypersonic weapons will become nuclear-capable or dual-capable systems can also seriously affect the nuclear relationship of the three countries. Last but not least, the possible end of the INF Treaty could create conditions for a new competition over medium- and intermediate-range land-based weapon systems. For example, if the United States starts to develop ground-based intermediate-range missiles and to deploy them near China in the Asia-Pacific region, China could become much more worried about the survivability of its nuclear weapon systems, even if the U.S. ground-based intermediate-range missiles are all conventionally armed.

Faced with this challenging landscape, would China be interested in pursuing arms control measures to limit the scale and scope of the competition? In theory, the withdrawal by the United States and Russia from existing arms control institutions would provide China with an opportunity to fill the gap in the leadership role vacated by the nuclear superpowers. A more open and proactive attitude toward arms control would bring China not only security and economic benefits in the process of promoting cooperative security and avoiding a costly arms race but also international reputation and diplomatic prestige by enhancing its image of a pacifist rising power. Some shape or form of a trilateral or multilateral nuclear arms control agreement would also create the image of China being an equal partner with the United States and Russia at the center stage of international strategic security issues.

In reality, however, there is still a long way before China would join an arms control effort. If the United States and Russia start to increase their nuclear arsenals post-New START, a growing fear in China about the credibility of its second-strike capability would likely make China less willing to be transparent about its own nuclear capabilities and future development and deployment plans. The lack of willingness to be more transparent would make any arms control effort a non-starter. The collapsing of a decades-long process of U.S.-Russia arms control efforts in the end would also work to convince China that the practice of arms control between big powers is a failed exercise and is not worth it for China to try. As of today, all leaders in the three countries seem to believe in the principle of negotiating from a position of strength. In the Chinese case, this would mean that China is unlikely to seriously consider arms control until it has possessed a capability somewhat similar or equal to the others in order to ensure that it would not be in a disadvantaged position in an arms control negotiation. There are no easy ways to change these deeply held views anytime soon. 

Given such possible consequences of a post-New START world, the top priority for the international community should be making every effort to preserve the existing arms control regime. If that effort does not succeed, the international community would face an unprecedented challenge in managing a complex trilateral nuclear arms competition, with no easy solution to set up new arms control institutions.

This piece was originally published by the Center for Naval Analyses.


1 For example, see: Riqiang Wu, “Merits of Uncertainty: The Evolution and Future of China's Nuclear Retaliatory Capability,” (Boston, MA: Project on Managing the Atom Seminar Series, Belfer Center for Science and International Affairs, Harvard University, 2018).

2 Keir A Lieber and Daryl G Press, “The New Era of Counterforce: Technological Change and the Future of Nuclear Deterrence,” International Security, 2017.

3 Hans M. Kristensen, Matthew McKinzie, and Theodore A. Postol, “How Us Nuclear Force Modernization Is Undermining Strategic Stability: The Burst-Height Compensating Super-Fuze,” Bulletin of the Atomic Scientists , March 1, 2017.

4 Vince Manzo, “Nuclear Arms Control without a Treaty? Risks and Options after New Start,” Center for Naval Analyses, March 2019.

5 Keir A. Lieber and Daryl G. Press, “U.S. Nuclear Primacy and the Future of the Chinese Deterrent,” China Security, no. Winter, 2007. However, it is necessary to point out that, under more realistic conditions, if the 3 B-2 bombers cannot hit all 20 silos simultaneously, China may have a potential opportunity to launch some of the missiles while the attack is still underway.

6 Manzo, “Nuclear Arms Control without a Treaty? Risks and Options after New Start.”

7 Shou, Xiaosong (寿晓松). 2013. The Science of Military Strategy(战略学), Military Science Press (军事科学出版社).

For example, see: Tong Zhao, “The Perception Gap in the Thaad Dispute--Causes and Solutions,” in China International Strategy Review 2017, ed. Jisi Wang (Beijing: International Strategic Institute, Peking University, November 2018).

9 Hans M. Kristensen and Robert S. Norris, “United States Nuclear Forces, 2018,” Bulletin of the Atomic Scientists (1974) 74, no. 2, 2018.

10 Lieber and Press, “The New Era of Counterforce: Technological Change and the Future of Nuclear Deterrence.”

11 “Editorial: “What the Fact That Trump Respects a Nuclear Superpower Russia Teaches Us (社评:特朗普敬重超级核大国俄罗斯的启示),” Global Times (环球时报), September 18,

12 Xiangli (孙向丽) Sun, “Strategic Choice in the Nuclear Age: On China's Nuclear Strategy (核时代的战略选择:中国核战略问题研究),” (Beijing: Center of Strategic Studies, Chinese Academy of Engineering Physics, 2013).

13 “Nuclear Posture Review,”  U.S. Department of Defense, February 2018.

14 “Editorial: Both China's Defense Spending and Strategic Nuclear Capablities Are Not Enough (社评:中国的军费和战略核力量都还不够),” Global Times (环球时报), December 14, 2016.

15 Hans M. Kristensen and Robert S. Norris, “Chinese Nuclear Forces, 2018,” Bulletin of the Atomic Scientists 74, no. 4, 2018.