Nop, a spontaneous murine dominant cataract mutation, was detected by slit lamp investigations and preliminary characterized as a nuclear opacity. Histological investigations confirmed these findings and revealed additionally polar cataracts with vacuolization. In contrast to wild-type lenses, the nuclei of the cortical cells could also be detected in the area of the lens nucleus in Nop lenses. No other pathological alterations were found in the eyes. Lens wet and dry weights, as well as the content of water-soluble lens proteins, were reduced in heterozygous and homozygous mutants. The body weight was only slightly altered, indicating a rather lens-specific growth retardation. Some parameters concerning the osmotic state of the lens were changed, however, only in the homozygous mutants. Electrophoresis of the water-soluble lens proteins of the mutants revealed either additional bands, not present in the wild types, or bands of overrepresented proteins only slightly present in wild-type lenses. The changes might be related to the reduced amount of gamma-crystallins, which alters the composition of lenticular proteins in the mutants. Northern blots probed with cDNA specific for alpha-, beta- or gamma-crystallin genes suggested a reduced transcription of the gamma-crystallin genes. In contrast, the transcription of alpha- and beta-crystallins appeared to be similar in wild type and the mutants. The selective reduced amount of gamma-crystallin specific RNA can be discussed as a biochemical indicator for the histologically observed changes of differentiation in the cataractous Nop lenses.