Cataractous mice (Cts/Cts homozygotes) were reciprocally crossed with 4 inbred strains of mice. In F1, expressivity of the gene fluctuated depending on the background genes. Expressivity of the gene in F1 was different from that in their F2, reflecting the heterogeneous genetic constitution of the latter. The Cts gene possessed complete penetrance, as might be expected of an autosomal semidominant gene. The initial sign of cataract formation was found at prenatal day 15 in the lens fibers near the anterior pole area of Cts/Cts homozygotes.