Although scientists dont completely understand why our cells eventually stop dividing, many of them believe that the shrinking of protective DNA caps on the end of chromosomes called telomeres may be responsible. This is based on the fact that every time a cell divides, a bit of telomere is lost until it is no longer active in adult tissue. Ronald DePinho, a molecular biologist and director of the Belfer Institute of the Dana-Farber Cancer Institute along with colleagues, successfully turned a number of mice into younger versions of themselves by stimulating the regeneration of certain tissues.