Disorders comprising a spectrum of brain malformations representing the paradigm of a diffuse neuronal migration disorder. They result in cognitive impairment; seizures; and HYPOTONIA or spasticity. Mutations of two genes, LIS1, the gene for the non-catalytic subunit of PLATELET-ACTIVATING FACTOR ACETYLHYDROLASE IB; and DCX or XLIS, the gene for doublecortin, have been identified as the most common causes of disorders in this spectrum. Additional variants of classical (Type I) lissencephaly have been linked to RELN, the gene for reelin, and ARX, the gene for aristaless related homeobox protein. (From Leventer, R.J., et al, Mol Med Today. 2000 Jul;6(7):277-84 and Barkovich, A.J., et al, neurology. 2005 Dec 27;65(12):1873-87.)