Cyclin-dependent kinase 5 (Cdk5) is a member of the family of cell cycle-related kinases. Previous neuropathological analysis ofcdk5−/−mice showed significant changes in CNS development in regions from cerebral cortex to brainstem. Among the defects in these animals, a disruption of the normal pattern of cell migrations in cerebellum was particularly apparent, including a pronounced abnormality in the location of cerebellar Purkinje cells. Complete analysis of this brain region is hampered in the mutant because most of cerebellar morphogenesis occurs after birth and thecdk5−/−mice die in the perinatal period. To overcome this disadvantage, we have generated chimeric mice by injection ofcdk5−/−embryonic stem cells into host blastocysts. Analysis of the cerebellum from the resultingcdk5−/−↔cdk5+/+chimeric mice shows that the abnormal location of the mutant Purkinje cells is a cell-autonomous defect. In addition, significant numbers of granule cells remain located in the molecular layer, suggesting a failure to complete migration from the external to the internal granule cell layer. In contrast to the Purkinje and granule cell populations, all three of the deep cerebellar nuclear cell groupings form correctly and are composed of cells of both mutant and wild-type genotypes. Despite similarities of thecdk5−/−phenotype to that reported inreelerandmdab-1−/−(scrambler/yotari) mutant brains,reelinanddisabled-1mRNA were found to be normal incdk5−/−brain. Together, the data further support the hypothesis that Cdk5 activity is required for specific components of neuronal migration that are differentially required by different neuronal cell types and by even a single neuronal cell type at different developmental stages.