RPGR and retinal degeneration: Our screening protocol identified Rd9 mice as a naturally-occurring model of human RP3, an X-linked retinal degeneration caused by mutations in RPGR. As in the majority of cases of RP3, and in a significant fraction of males with simplex RP [3], retinal degeneration in Rd9 mice is caused by a mutation in the alternatively-spliced exon ORF15.