Mutations in the Unc5h3 gene, a receptor for the netrin 1 ligand, result in abnormal migrations of both Purkinje and granule cells to regions outside the cerebellum and of granule cells to regions within the cerebellum. Because both Purkinje and granule cells express this molecule, we sought to determine whether one or both of these cell types are the primary target of the mutation. Chimeric mice were made between wild-type ROSA26 transgenic mouse embryos (whose cells express beta-galactosidase) and Unc5h3 mutant embryos. The resulting chimeric brains exhibited a range of phenotypes. Chimeras that had a limited expression of the extracerebellar phenotype (movement of cerebellar cells into the colliculus and midbrain tegmentum) and the intracerebellar phenotype (migration of granule cells into white matter) had a normal-appearing cerebellum, whereas chimeras that had more ectopic cells had attenuated anterior cerebellar lobules. Furthermore, the colonization of colliculus and midbrain tegmentum by cerebellar cells was not equivalent in all chimeras, suggesting different origins for extracerebellar ectopias in these regions. The granule cells of the extracerebellar ectopias were almost entirely derived from Unc5h3/Unc5h3 mutant embryos, whereas the ectopic Purkinje cells were a mixture of both mutant and wild-type cells. Intracerebellar ectopias in the chimera were composed exclusively of mutant granule cells. These findings demonstrate that both inside and outside the cerebellum, the granule cell is the key cell type to demarcate the boundaries of the cerebellum.