Aspm specifically maintains symmetric proliferative divisions of neuroepithelial cells:
…Our results demonstrate that Aspm is crucial for maintaining a cleavage plane orientation that allows symmetric, proliferative divisions of NE cells during brain development. These data provide a cell biological explanation of the primary microcephaly observed in humans with mutations in ASPM, which also has implications for the evolution of mammalian brains.
Update: Previous commentary on ASPM.