PLATELET-ACTIVATING factor (PAF) is involved in a variety of biological and pathological processes(1) and PAF acetylhydrolase, which inactivates PAF by removing the acetyl group at the sn-2 position, is widely distributed in plasma and tissue cytosols(2,3). One isoform of PAP acetylhydrolase present in bovine brain cortex is a heterotrimer comprising subunits with relative molecular masses of 45K, 30K and 29K (ref. 4). We have now isolated the complementary DNA for the 45K subunit. Sequence analysis revealed a striking identity (99%) of the subunit with a protein encoded by the causative gene (LIS-1) for MiUer-Dieker lissencephaly(5), a human brain malformation manifested by a smooth cerebral surface and abnormal neuronal migration. This indicates that the LIS-1 gene product is a human homologue of the 45K subunit of intracellular PAF acetylhydrolase. Our results raise the possibility that PAF and PAF acetylhydrolase are important in the formation of the brain cortex during differentiation and development.