Classical Lissencephalies and Subcortical Band Heterotopias: 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.)Lissencephaly: A "smooth brain" malformation of the CEREBRAL CORTEX resulting from abnormal location of developing neurons during corticogenesis. It is characterized by an absence of normal convoluted indentations on the surface of the brain (agyria), or fewer and shallower indentations (pachygryia). There is a reduced number of cortical layers, typically 4 instead of 6, resulting in a thickened cortex, and reduced cerebral white matter that is a reversal of the normal ratio of cerebral white matter to cortex.Neuronal Migration Disorders: Disorders resulting from defects in migration of neuronal cells during neurogenesis. Developing nerve cells either fail to migrate or they migrate to incorrect positions resulting in formation of heterotopias, lissencephaly, or other malformations and dysfunctions of the nervous system.Cobblestone Lissencephaly: The smooth pebbled appearance of the CEREBRAL CORTEX with a thickened cortex and reduced and abnormal white matter, which results from migration of heterotopic neurons beyond the marginal zone into the leptomeninges through gaps in the external BASEMENT MEMBRANE. There is also enlarged ventricles, underdeveloped BRAINSTEM and cerebellum, and absence of the CORPUS CALLOSUM. These abnormalities occur as a syndrome without other birth defects (cobblestone complex) or in other syndromes associated with congenital MUSCULAR DYSTROPHY, often involving the eye, such as the Walker-Warburg Syndrome, Fukuyama congenital muscular dystrophy, and muscle-eye-brain disease.Neuropeptides: Peptides released by NEURONS as intercellular messengers. Many neuropeptides are also hormones released by non-neuronal cells.Choristoma: A mass of histologically normal tissue present in an abnormal location.Periventricular Nodular Heterotopia: A disorder resulting from a defect in the pattern of neuronal migration in which ectopic collections of neurons lie along the lateral ventricles of the brain or just beneath, contiguously or in isolated patches.Nervous System Malformations: Structural abnormalities of the central or peripheral nervous system resulting primarily from defects of embryogenesis.