A water-soluble protein (called “33-kDa protein”) that exhibits light-dependent phosphorylation has been shown to be a major protein of mammalian rod photoreceptors. Although the function of this protein is unknown, it has been implicated in the biochemical cascade mediating the rod visual response. Using a retinal cDNA from the rat and somatic cell hybrids, we have mapped the gene corresponding to this protein to mouse Chromosome 1 and, by analyzing the progeny of an intersubspecific backcross, have positioned it near Lamb2 (the β2 chain of laminin). We have designated the gene Rpr-1 (rod photoreceptor protein-1).