In the embryonic central nervous system of the fruit fly Drosophila, most neurons and glial cells are generated by asymmetric division of neural stem cells called neuroblasts. Several genes have been identified that are required for the establishment of neuroblast polarity, for the asymmetric segregation of cell fate determinants and for the proper orientation and geometry of the mitotic spindle. However, little was known about the interactions between these genes and their respective gene products. It has emerged that most of the relevant proteins are assembled into three major protein complexes whose molecular interactions are conserved in evolution.