[show abstract][hide abstract] ABSTRACT: Whereas neurons of the lower layers (VI-V) of the cerebral cortex are first born from dividing precursors at the ventricular zone, upper layer neurons (II-IV) subsequently arise from divisions of intermediate neuronal precursors at the subventricular zone (SVZ). Little is known about mechanisms that control the proliferation of SVZ neuronal precursors. We herein report that the restricted expression of the homeodomain transcription factor Cux-2 in the SVZ regulates the proliferation of intermediate neuronal precursors and the number of upper layer neurons. In Cux-2-deficient mice (Cux-2-/-), there is excessive number of upper layer neurons and selective expansion of SVZ neuronal precursors. Double-labeling experiments demonstrate that Cux-2-/- upper layer precursors reenter the cell cycle in a higher frequency than wild-type precursors. Overexpression studies indicate that Cux-2 controls cell cycle exit in a cell-autonomous manner. Analysis of Cux-1-/-; Cux-2-/- double mutant revealed that Cux-2 controls SVZ proliferation independently of Cux-1, demonstrating that this is a unique function of Cux-2, not redundant with Cux-1 activities. Our results point to Cux-2 as a key element in the control of the proliferation rates of the SVZ precursors and the number of upper cortical neurons, without altering the number of deep cortical layers.