Since the number of precursors and their mode of division critically influence final cortical size and complexity, the neural precursor state is a focal point for regulation in brain development. Neural precursors must downregulate the pluripotency gene network promoted in ES cells while remaining proliferative; they must also suppress the neuronal differentiation program until mitotic exit. The chromatin landscape associated with this particular state requires the neural progenitor-specific BAF (npBAF) complex (Lessard et al., 2007). In contrast to esBAF, npBAF can have alternative subunits at a number of positions, including either Brg1 or Brm as the core ATPase; either BAF250a or BAF250b; and either a homodimer of BAF155 or a heterodimer of BAF155 and BAF170 (Figure 1). Three of the subunits which are unchanged during neural induction – BAF53a, SS18 and BAF45a/d – are also important for specifying the undifferentiated, proliferative state of the progenitors; as we will discuss, these three positions later undergo switching around the time of mitotic exit, such that the postmitotic neuronal BAF (nBAF) contains BAF53b, CREST and BAF45b/c, at those positions, respectively.
