Breaking the cell cycle of HSCs by p57 and friends.

The cell cycle regulators involved in maintaining the quiescence, and thereby the self-renewal capacity, of somatic stem cells have long been elusive. Two new Cell Stem Cell articles in this issue (Matsumoto et al., 2011; Zou et al., 2011) now show that the CDK inhibitor p57 is a crucial brake for cycling HSCs, and links self-renewal activity to cell cycle quiescence.