a guanine nucleotide exchange factor (GEF) that plays a role in cilia biogenesis and maintenance. Interacts with the GDP-bound form of the small GTPase RAB8A. Promotes the exchange of GDP to GTP, converting inactive GDP-bound Rab proteins into their active GTP-bound form. Probably regulates cilia formation by regulating actin stress filaments and cell contractility. Plays an important role in photoreceptor integrity. May play a critical role in spermatogenesis and in intraflagellar transport processes May be involved in microtubule organization and regulation of transport in primary cilia. Interacts with PDE6D, RPGRIP1, CEP290, and DFNB31. Defects in RPGR are the cause of retinitis pigmentosa (RP3, RP15, and RPDSI), cone-rod dystrophy, X-linked 1 (CORDX1), and macular degeneration, X-linked, atrophic (MDXLA). Six isoforms of the human protein are produced by alternative splicing. Isoform 3 is found only in the retina. Colocalizes with RPGRIP1 in the outer segment of rod photoreceptors and cone outer segments. Isoform 6 associates with microtubule organizing centers, cilium basal bodies, and cilium axonemes. Interacts with NPM1 (via C-terminus), RPGRIP1L, SMC1A and SMC3. Note: This description may include information from UniProtKB.