Zic genes encode zinc finger proteins, the expression of which is highly restricted to cerebellar granule cells and their precursors. These genes are homologs of the Drosophila pair-rule gene odd-paired. To clarify the role of the Zic1 gene, we have generated mice deficient in Zic1. Homozygous mice showed remarkable ataxia during postnatal development. Nearly all of the mice died within 1 month. Their cerebella were hypoplastic and missing a lobule in the anterior lobe. A bromodeoxyuridine labeling study indicated a reduction both in the proliferating cell fraction in the external germinal layer (EGL), from 14 d postcoitum, and in forward movement of the EGL. These findings suggest that Zic1 may determine the cerebellar folial pattern principally via regulation of cell proliferation in the EGL.