The autosomal recessive mouse mutation reeler leads to impaired motor coordination, tremors and ataxia. Neurons in affected mice fail to reach their correct locations in the developing brain, disrupting the organization of the cerebellar and cerebral cortices and other laminated regions. Here we use a previously characterized reeler allele (rl(tg)) to close a gene, reelin, deleted in two reeler alleles. Normal but not mutant mice express reelin in embryonic and postnatal neurons during periods of neuronal migration. The encoded protein resembles extracellular matrix proteins involved in cell adhesion. The reeler phenotype thus seems to reflect a failure of early events associated with brain lamination which are normally controlled by reelin.