Human Phenotype Ontology entry HP_0001339 (Lissencephaly) is a spectrum of malformations of cortical development caused by insufficient neuronal migration that subsumes the terms agyria, pachygyria and subcortical band heterotopia. See also neuropathological definitions for 2-, 3-, and 4-layered lissencephaly. Also known as: Fewer or absent grooves in brain.