USH2A and retinitis pigmentosa 1: Moreover, many genes for RP are causative of a small percentage of cases, with the exception of the rhodopsin gene (present in about 20-25% of AD-RP), USH2A gene (present in about 15-20% of AR-RP especially including many Usher syndrome type II), and RPGR gene (present in about 65-70% of XL-RP) [6, 42, 45].