Muscle stem cells, called satellite cells, are responsible for muscle growth and repair throughout life. Different subsets of satellite cells have varying degrees of self-renewal and differentiation potential, but how and when these different subsets arise has not been addressed in vivo. Now, on p. 1649, Andrew Brack and colleagues analyse the precise timing of phenotypic and functional divergence of different satellite cell subpopulations in mouse muscle. The authors use a genetic approach to label satellite cells with an inducible reporter, which becomes diluted with every round of cell division. In this way, the authors identify label-retaining cells (LRCs) that possess greater self-renewal potential than non-LRCs, which are prone to differentiation. The LRCs emerge shortly after birth, become functionally distinct at later stages of postnatal muscle maturation, and are re-established after injury. By comparing slow and fast dividing cells, the authors identify the cell cycle inhibitor p27kip1 as a novel regulator of LRCs, required to maintain their self-renewal potential.