Schematic representation of the lineage relationships of the cell types examined in these studies. Pluripotent neuroepithelial stem cells (NSC) give rise to glial-restricted precursor (GRP) cells and neuron-restricted precursor (NRP) cells. NRP cells can give rise to multiple populations of neurons, whereas GRP cells give rise to astrocytes and oligodendrocyte-type-2 astrocytes (O-2A/OPCs). The O-2A/OPCs in turn give rise to oligodendrocytes. The progenitor cells that lie between NSCs and differentiated cell types, and are the major dividing cell population in the CNS, appear to be exceptionally vulnerable to the effects of chemotherapeutic agents. Also sharing this vulnerability are nondividing oligodendrocytes.