Fig. 7. Schematic representation of some proteins normally found in the rod photoreceptor outer segment, interphotoreceptor matrix, and retinal pigment epithelium. Genes encoding these proteins are considered candidate genes, because mutations in them could result in compromise of the phototransduction cascade or the mechanism by which vitamin A is transported between the photoreceptors and the pigment epithelium with consequent photoreceptor cell degeneration. IRBP, interphotoreceptor retinoid binding protein; CRBP, cellular retinol binding protein; CRALBP, cellular retinal binding protein; PDE, phosphodiesterase; cGMP, cyclic guanosine monophosphate. (Modified from Berson EL. Retinitis pigmentosa. The Friedenwald Lecture. Invest Ophthalmol Vis Sci 1993;34:1659.)