RPGR encodes a protein with a series of six RCC1-like domains, characteristic of the highly conserved guanine nucleotide exchange factors. Mutations in this gene have been associated with X-linked retinitis pigmentosa (XLRP). Multiple alternatively spliced transcript variants that encode different isoforms of this gene have been reported, but the full-length natures of only some have been determined.The predicted 90-kD RPGR protein contains in its N-terminal half a tandem repeat structure highly similar to the regulator of chromosome condensation (RCC1), which regulates the GTPase RAN. The C terminus of the protein contains a cluster of basic residues followed by a consensus isoprenylation site. The authors noted that confirmation of the isoprenylation of this site would establish a novel means of membrane anchorage for a GTPase regulator.