Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most prominently, they are found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade. Another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in vision. Humans have in total nine opsins. Beside vision and light perception, opsins may also sense temperature, sound, or chemicals. Structure and function Animal opsins detect light and are the molecules that allow us to see. Opsins are G-protein-coupled receptors (GPCRs), which are chemoreceptors and have seven transmembrane domains forming a binding pocket for a ligand. The ligand for opsins is the vitamin A-based chromophore 11-cis-retinal, which is covalently bound to a lysine residue in the seventh transmembrane domain through a Schiff-base. However, 11-cis-retinal only blocks the binding pocket and does not activate the opsin. The opsin is only activated when 11-cis-retinal absorbs a photon of light and isomerizes to all-trans-retinal, the receptor activating form, causing conformal changes in the opsin, which activate a phototransduction cascade. Thus, a chemoreceptor is converted to a light or photo(n)receptor. In the vertebrate photoreceptor cells, all-trans-retinal is released and replaced by a newly synthesized 11-cis-retinal provided from the retinal epithelial cells. Beside 11-cis-retinal (A1), 11-cis-3,4-didehydroretinal (A2) is also found in vertebrates as ligand such as in freshwater fishes. A2-bound opsins have a shifted λmax and absorption spectrum compared to A1-bound opsins. Functionally conserved residues and motifs The seven transmembrane α-helical domains in opsins are connected by three extra-cellular and three cytoplasmic loops. Along the α-helices and the loops, many amino acid residues are highly conserved between all opsin groups, indicating that they serve important functions and thus are called functionally conserved residues. Actually, insertions and deletions in the α-helices are very rare and should preferentially occur in the loops. Therefore, different G-protein-coupled receptors have different length and homologous residues may be in different positions. To make such positions comparable between different receptors, Ballesteros and Weinstein introduced a common numbering scheme for G-protein-coupled receptors. The number before the period is the number of the transmembrane domain. The number after the period is set arbitrarily