ATR and microcephaly: Interestingly, whereas deletion of Nbs1 in the CNS affects only the survival of proliferating progenitors, Atr knockout causes cell death in both proliferating progenitors and postmitotic neurons of the cortex, leading to much stronger microcephaly in Atr-CNSΔ mice than Nbs1-CNSΔ mice (Zhou et al., 2012).