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This article was last updated on February 12, 2020, at NEJM.org.

We thank Drs. Michel Bernier and Anne E. Burke for valuable input, Mr. Marc Raley for work on previous versions of the figures, Dr. David G. Le Couteur for assistance with the preparation of an earlier version of the manuscript, and the Intramural Research Program of the National Institute on Aging, National Institutes of Health, for its support.