Fig. P1.

Cortical progenitor cells (circles) generate deep-layer neurons (red) early in development, followed by upper-layer neurons (blue) late in development. (Upper) We show that progenitor cells express Ikaros (green) at early time points when they are producing deep-layer neurons. (Lower) Sustained expression of Ikaros in cortical progenitor cells beyond its normal period results in a prolonged and increased production of deep-layer neurons coupled to a delayed onset of and decreased production of upper-layer neurons.