Abstract To study the effect of a reduced granule cell number on the development of the cerebellum, mice were administered with methylazoxymethanol (MAM) at birth and examined histologically at 21 days of age. In these mice, green fluorescent protein had been knocked in within the glutamic acid decarboxylase 67 gene, resulting in fluorescent GABAergic neurons. In severe cases, granule cells were greatly reduced in number and mixed with Purkinje cells instead of forming layers. In about 10% of the MAM-treated mice, an ectopic cell layer consisting of granule cells was observed within the molecular layer. Concomitantly, basket cells disappeared. The transient interruption of granule cell production in the external granular layer presumably resulted in their incomplete migration through the molecular layer.