Mutations in the retinal degeneration, retinal degeneration slotilperipherin) and rhodopsin genes cause photoreceptor degeneration in humans and mice. Although the phenotypes arising from these mutations are different, suggesting different mechanisms of pathogenesis, we present evidence that apoptosis may be the final common pathway of the disease process linking genotype to phenotype. We observed internucleosomal cleavage of retinal DNA by gel electrophoresis and fragmented DNA at the single cell level by labeling the nicked DNA ends with biotinylated poly (dU). In retinal degeneration mice, DNA fragmentation occurred during the period of photoreceptor degeneration. In retinal degeneration slow mice and in transgenic mice expressinga mutant (Pro347Ser) rhodopsin gene, DNA fragmentation occurred after normal histogenetic cell death (also apoptosis) had ceased. Since DNA fragmentation by internucleosomal cleavage is a cardinal feature of apop tosis, our data suggest that all three of these genetic mutations lead to apoptosis.