UCLA researchers examined muscle stem cells in mice and found that NDRG1 protein increased dramatically in older cells, to a level 3.5 times higher than in young cells.[1] This protein acts as a brake that suppresses the mTOR signaling pathway, slowing cell activation and muscle repair after injury, but increasing their survival and endurance.[1] When researchers blocked NDRG1 in older mice, roughly the equivalent of 75 human years, muscle stem cells behaved like young ones, quickly activating and speeding up healing after injury.[1] However, this change came with a downside: without the protection of NDRG1, fewer stem cells survived over time, limiting muscle regeneration after repeated injuries.[1] According to scientists, the increased level of NDRG1 arises through "cell survivorship bias" - cells with a lack of NDRG1 die, slower but more resistant cells remain.[1] The results were confirmed by studies on cells from young and old mice in laboratories and in living tissues.[1]