Classical lissencephaly and double cortex are genetic neuronal migration disorders associated with mental retardation and epilepsy. In classical lissencephaly, the six-layered cortex is replaced by a four layered structure lacking normal gyri or sulci. In double cortex, a second layer of cortical neurons underlies a normal cortex. A mutation in LIS1 or doublecortin can lead to either classical lissencephaly or double cortex, but because LIS1 is autosomal and doublecortin is X-linked (on the X chromosome), the disease inheritance pattern and risk of recurrence for the two genes are distinct. Mutation analysis for LIS1 and doublecortin is essential in determining the etiology of the disease in patients and may be helpful in determining the recurrence risk in families.