A conditional deletion of Cdc42 in the developing cortex of mice at E9.5 causes mislocalization of adherens junctions due to missing cellular polarization, usually maintained by Cdc42-Par complex interaction (Cappello et al., 2006) and a telencephalon-specific knockout leads to holoprosencephaly—a incomplete bifurcation of cerebral hemispheres (Chen et al., 2006). The gene discussed is CDC42; the disease is holoprosencephaly.