The protein, called GDF-11, was injected into old mice. Old mice, much like humans, develop thickened heart walls as they grow older. The heart was reduced in size and thickness, resembling the healthy hearts of younger mice. A finding by Richard T. Lee, a Harvard Medical School professor at Brigham and Women's Hospital and Amy Wagers, a professor in Harvard's Department of Stem Cell and Regenerative Biology, ultimately may rewrite our understanding of aging.