Development of the central nervous system requires the coordinated interplay of intrinsic and extrinsic factors. I examined the actions of an extrinsic factor, TWEAK, and an intrinsic factor, Iroquois1 (Irx1) on neural progenitor cell behavior. Embryonic day 12 (E12) and post-natal day 1 (PN1) neural progenitor cells both express Fn14,...