This mutation, identified in a fundus screen of progeny of an ENU mutagenized male mouse, comprises a T to C transition at nucleotide position 553, in the third of the gene's 5 exons. This results in replacement of cysteine by arginine at amino acid 185 of the protein (C185R), in the extracellular, intradiscal loop E-II between transmembrane domains IV and V. The level of rhodopsin is reduced in the retinas of homozygous and, to a lesser extent, of heterozygous mutant mice.