A lot of what goes on during aging remains too poorly understood for straightforward translation. There are hallmarks of aging, and researchers are getting a handle on its biological mechanisms. But in a basic sense, “we still don’t have much of an idea what causes aging,” said Björn van Eyss of the Leibniz Institute for Aging Research. Part six of BioWorld’s multipart series on extending the human lifespan explores the moonshot attracting the most attention: in vivo partial reprogramming.