Knockout studies have shown that the polycomb gene Bmi-1 is important for 
postnatal, but not embryonic, neural stem cell (NSC) self-renewal and have 
identified the cell-cycle inhibitors p16/p19 as molecular targets. Here, using 
lentiviral-delivered shRNAs in vitro and in vivo, we determined that Bmi-1 is 
also important for NSC self-renewal in the embryo. We found that neural 
progenitors depend increasingly on Bmi-1 for proliferation as development 
proceeds from embryonic through adult stages. Acute shRNA-mediated Bmi-1 
reduction causes defects in embryonic and adult NSC proliferation and 
self-renewal that, unexpectedly, are mediated by a different cell-cycle 
inhibitor, p21. Gene array analyses revealed developmental differences in 
Bmi-1-controlled expression of genes in the p21-Rb cell cycle regulatory 
pathway. Our data therefore implicate p21 as an important Bmi-1 target in NSCs, 
potentially with stage-related differences. Understanding stage-related 
mechanisms underlying NSC self-renewal has important implications for 
development of stem cell-based therapies.