Recently, several genes that regulate the development of the cerebral cortex and are potential pharmacological targets have been cloned. Reelin, an extracellular matrix glycoprotein secreted by Cajal-Retzius cells in the marginal zone, instructs the radial organization of the cortical plate. The response of cortical plate cells to reelin requires the tyrosine kinase adaptor disabled-1 (Dab1). Cyclin-dependent kinase 5 and its activator p35 are necessary for the development of the cortical plate, probably at a later stage than reelin/Dab1. The transcription factor Tbr-1 is essential for differentiation of preplate and Cajal-Retzius cells and for formation of thalamocortical connections, while D1x-1/2 are required for tangential migration. Some neurotrophin systems such as neurotrophin 4, brain-derived neurotrophic factor, and neuregulin and its receptor ErbB are also thought to assist in the regulation of cortical development. In addition, a few genes implicated in human cortical dysplasias have been characterized. LIS1 encodes a protein related to platelet-activating factor acetyl hydrolase that is defective in lissencephaly-1 of the Miller-Dieker type, while the double cortex malformation is related to mutations of a new gene dubbed doublecortn.