DCX and classic lissencephaly: Genetic studies in mice and humans have identified some of the molecular determinants of neuronal migration: mutations in doublecortin (DCX) (des Portes et al., 1998, Gleeson et al., 1998) and LIS1 (Reiner et al., 1993) have been shown to impair migration and cause type 1 lissencephaly in humans, a disease characterized by a four-layered cortex with an absence or diminution of gyri and sulci (Dobyns and Truwit, 1995).