To date, six genetic loci have been mapped on human X-chromosome (see the Retinal Information Network [RetNet] at https://sph.uth.edu/retnet/ and the Human Gene Mutation Database [HGMD] at http://www.hgmd.cf.ac.uk) but, according to the linkage analyses, RP-associated mutations are recognized in two main genes: retinitis pigmentosa GTPase regulator (RPGR or RP3; OMIM 312610) and retinitis pigmentosa 2 (RP2; OMIM 312600). This evidence concerns the gene RP2 and retinitis pigmentosa 1.