Rhodopsin is the photoreceptor in the retinal rods. It is activated by photons, transduces visual information through its cognate G protein, transducin, and is inactivated by arrestin binding . Using atomic-force microscopy, rhodopsin was found to be arranged into paracrystalline arrays of dimers in mouse disc membranes. Rhodopsin is considered to be the prototype of G protein-coupled receptors (GPCRs), and is the first GPCR for which a crystal structure was solved. Research studies have linked mutations in the gene encoding rhodopsin to retinitis pigmentosa , a disease characterized by retinal degeneration resulting in reduced peripheral vision and night blindness.