Purkinje cells of different molecular phenotypes subdivide the cortex of the cerebellum both rostrocaudally into parasagittal bands and mediolaterally into transverse zones. Superimposed on the Purkinje cell compartmentation, the cerebellar cortex is pleated into a reproducible array of lobes and lobules. During cerebellar development, Purkinje cell bands are formed through the rostrocaudal dispersal of embryonic clusters, triggered primarily by a Reelin-dependent signaling pathway. In the naturally occurring mouse mutant cerebellar deficient folia (cdf), there is a failure of Purkinje cell dispersion that results in widespread Purkinje cell ectopia in the adult. The ectopia is restricted primarily to that subset of Purkinje cells that does not express zebrin II/aldolase C and that forms ectopic clusters in among the cerebellar nuclei. Most Purkinje cells that express zebrin II are located normally in a monolayer. Thus, the cerebellum of cdf mutants has a failure of Purkinje cell dispersion that is confined primarily to a zebrin II-negative (zebrin II(-)) subpopulation. Despite the Purkinje cell ectopia, the parasagittal band organization of the cerebellum is still clear. The shortening of the cortex is distributed evenly over all lobules, with the result that transverse expression boundaries are relocated with respect to the lobules and fissures. The number of Purkinje cells in the cdf/cdf cerebellum is similar to the number in littermate controls. Therefore, it appears that the lesion in cdf results in the failure of a zebrin II(-) Purkinje cell subset to disperse either due to a cell intrinsic defect or due to an abnormal interaction between the Purkinje cells and either granule cells or afferent inputs.