In muscles, this happens because muscle stem cells no longer function properly, making muscle regrowth harder. The study, conducted in mice, reveals that aging muscle stem cells accumulate higher levels of a protein that slows their ability to regrow and repair muscle when needed. When scientists compared muscle stem cells from young and old mice, they discovered that a gene called NDRG1 plays a role. In older muscle, this gene’s protein levels were about 3.5 times higher than in young muscle. Without NDRG1’s protection, only a few muscle stem cells survived.