Neural progenitor cells must be kept alive throughout development to enable neurogenesis to proceed normally, but the mechanisms controlling the self-renewal of stem cells are poorly understood. In the October 31 Nature, Petur Petersen and colleagues report critical roles for numb and numblike, mammalian homologues of Drosophila numb in maintaining progenitor populations during mouse embryogenesis (Nature, 419:929-934, October 31, 2002).

Petersen et al. examined Numb function in mammalian neurogenesis using a transgenic mouse line in which Cre protein expression is driven by the nestin promoter, which is active in neural progenitor cells and somites. Mice lacking either numb or numblike in the nervous system were born normally, but the double deletion of both genes resulted in embryonic lethality (around E11.5). In these embryos, neurons differentiate early during development, but there is a dramatic reduction in the number of proliferating precursor cells. As development proceeds the progenitor...