A novel ENU-induced mutation in the mouse leading to a nuclear and zonular opacity of the eye lens (Aey1) was mapped to chromosome 1 between the markers D1Mit303 and D1Mit332. On the basis of the chromosomal position, the gamma -crystallin encoding gene cluster (Cryg) and the beta A2-crystallin encoding gene Cryba2 were tested as candidate genes. An A --> T mutation destroys the start codon of the Cryge gene in the mutants; this mutation was confirmed by the absence of a restriction site for NcoI in the corresponding genomic fragment of homozygous mutants. The next in-frame start codon is 129 bp downstream; this predicted truncated gammaE-crystallin consists of 131 amino acids, resulting in a molecular mass of 14 kD. However, another open reading frame was observed just 19 bp do downstream of the regular Cryge start codon, resulting in a protein of 119 amino acids and a calculated molecular weight of 13 kD. Western blot analysis using polyclonal antibodies against gamma -crystallins or the novel Aeyl-specific protein demonstrated the specific expression of the Aeyl protein in the cataractous lenses only; the truncated form of the gammaE-crystallin could not be detected. Therefore, it is concluded that the novel protein destroys the sensitive cellular structure of the eye lens.