To elucidate the regulatory mechanisms underlying lens development, we searched for members of the large Maf family, which are expressed in the mouse lens, and found three, c-Maf, MafB, and Nrl. Of these, the earliest factor expressed in the lens was c-Maf. The expression of c-Maf was most prominent in lens fiber cells and persisted throughout lens development. To examine the functional contribution of c-Maf to lens development, we isolated genomic clones encompassing the murine c-maf gene and carried out its targeted disruption. Insertion of the β-galactosidase (lacZ) gene into the c-maf locus allowed visualization of c-Maf accumulation in heterozygous mutant mice by staining for LacZ activity. Homozygous mutant embryos and newborns lacked normal lenses. Histological examination of these mice revealed defective differentiation of lens fiber cells. The expression of crystallin genes was severely impaired in the c-maf-null mutant mouse lens. These results demonstrate that c-Maf is an indispensable regulator of lens differentiation during murine development.