Concurrently with patterning in the developing dorsal telencephalon, NSCs expand in number. Through a developmental switch not yet fully understood, after the majority of this expansion has occurred, stem cells then begin to generate neuronal precursors in a neurogenic phase that lasts for approximately 6 days in rodents and 10–12 weeks in primates (Caviness et al., 1995; Rakic, 1995) (Figure 1). Cortical excitatory neurons are derived from NSC that line the dorsal telencephalic ventricle. The primary stem cells in this ventricular zone (VZ) are called radial glia because of their expression of glial markers such as GFAP and GLAST, and their cellular morphology. Radial glial cells have an apical end foot attachment at the ventricle, a cell body that is near the ventricle, and a long radial process that is attached at the pial surface (Levitt et al., 1981). Radial glia can undergo self-renewing cell divisions, or asymmetric cell divisions that directly give rise to neurons (Noctor et al., 2001). Another product of radial glial division are committed neurogenic progenitors that migrate to the subventricular zone (SVZ), above the VZ, where they in turn proliferate to give rise to neurons. The committed neuronal progenitors of the SVZ, referred to as intermediate progenitor cells (IPCs) express the transcription factor TBR2 and lack the self-renewal properties of true stem cells (Pontious et al., 2008). However, their proliferation is important for the expansion of cortical layers, as demonstrated by the decrease in cortical surface area and thickness in mice lacking tbr2 (Arnold et al., 2008; Sessa et al., 2008).