During a large-scale ENU mutagenesis screen, a mouse mutant with a dominant cataract was detected and referred to as Aey4. Aim of this studs was the morphological description of the mutant. the mapping of the mutation. and the characterization of the underlying molecular lesion. The slit-lamp examination revealed a strong nuclear cataract surrounded by a homogeneous milky opacity in the inner cortex. The histological analysis demonstrated remnants of cell nuclei throughout the entire lens. The mutation was mapped to Chromosome 1 by a genome-wide linkage making the six gamma-crystallin encoding genes and the closely linked betaA2-crystallin encoding gene to relevant candidate genes. Finally, a T-->A exchange in exon 2 of the gammaD-crystallin encoding gene (symbol: Crygd) was demonstrated to be causative for the cataract phenotype: this particular mutation is. therefore, referred to Crygo(Aey4) . The alteration in codon 76 leads to an amino acid exchange of Val-->Asp. Val at this position is highly conserved: it is found in all mouse and rat gammaD/E/F-crystallins as well as in the human gammaA- and gammaD-crystallins. It may, be replaced solely by Ile. which is present in all bovine gamma-crystallins, in the rat and mouse gammaA/B/C-crystallins, as well as in the human gammaB/C-crystallins. It is predicted that the exchange of a hydrophobic side chain by a polar and acidic one might influence the microenvironment by a dramatic decrease of the isoelectric point by 1.5 pH units in the 10 amino acids surrounding position 76. The Crygd(Aey4) additionally demonstrates the importance of the integrity of the Cryg gene cluster for lens transparency.