A novel protein, called RPGRIP, has been identified as interacting with the RPGR protein, which is mutated in a severe form of human retinal degeneration, X-linked retinitis pigmentosa (RP3 type). The bovine RPGRIP was identified initially by screening for RPGR-interacting proteins with a bovine retina cDNA library using the yeast two-hybrid system. The specificity of the interaction was confirmed by co-immunoprecipitation of in vitro translated protein and using RPGR mutants. The human RPGRIP gene was isolated and shown to be expressed in retina and testis. Human RPGRIP spans a genomic interval of 34 kb, and consists of 15 exons, some of which are alternatively spliced. It was mapped using monochromosomal and radiation hybrid cell lines to chromosomal region 14q11. The function of RPGRIP is unknown; it shows no homology to proteins of known function, although it is predicted to form two coiled-coil domains at the N-terminus. RPGRIP is a strong candidate gene for causing human retinal degeneration.