PAX6 and microcephaly: In the neocortex, loss of Pax6 function results in microcephaly, abnormal development of the secondary progenitor population of the subventricular zone (SVZ, also known as basal progenitor cells, BP cells) and a disproportionate reduction in the production of later-born, upper layer neurons [12], [14], [15], [17], [20]–[23].