The adult heart's inability to repair itself after injury has long stood as one of medicine's most stubborn limitations — a consequence of the very maturity that makes the heart reliable. Researchers at Kyoto University and the University of Osaka have now identified a protein called PRDM16 that governs this ancient biological bargain, acting as a molecular dimmer between a heart cell's capacity to divide and its capacity to function. The discovery, published in Stem Cell Reports, does not yet offer a cure, but it names the gatekeeper of a transition that has shaped human vulnerability to hear