of the rhabdomeric opsins (rhabopsins, r-opsins) and functionally and structurally a rhabopsin, but does not occur in rhabdomeres. Tetraopsins The tetraopsins include the neuropsins, the Go-opsins, and the chromopsins. The chromopsins consist of seven subgroups: the RGR-opsins, the retinochromes, the peropsins, the varropsins, the astropsins, the nemopsins, and the gluopsins. Neuropsins Neuropsins are sensitive to UVA, typically at 380 nm. They are found in the brain, testes, skin, and retina of humans and rodents, as well as in the brain and retina of birds. In birds and rodents they mediate ultraviolet vision. They couple to Gi-proteins. In humans, Neuropsin is encoded by the OPN5 gene. In the human retina, its function is unknown. In the mouse, it photo-entrains the retina and cornea at least ex vivo. Go-opsins Go-opsins are absent from higher vertebrates and ecdysozoans. They are found in the ciliary photoreceptor cells of the scallop eye and the basal chordate amphioxus. In Platynereis dumerilii however, a Go-opsin is expressed in the rhabdomeric photoreceptor cells of the eyes. RGR-opsins RGR-opsins, also known as Retinal G protein coupled receptors are expressed in the retinal pigment epithelium (RPE) and Müller cells. They preferentially bind all-trans-retinal in the dark instead of 11-cis-retinal. RGR-opsins were thought to be photoisomerases but instead, they regulate retinoid traffic and production. In particular, they speed up light-independently the production of 11-cis-retinol (a precursor of 11-cis-retinal) from all-trans-retinyl-esters. However, the all-trans-retinyl-esters are made available light-dependently by RGR-opsins. Whether RGR-opsins regulate this via a G-protein or another signaling mechanism is unknown. The cattle RGR-opsin absorbs maximally at different wavelengths depending on the pH-value. At high pH it absorbs maximally blue (469 nm) light and at low pH it absorbs maximally UV (370 nm) light. Peropsin Peropsin, a visual pigment-like receptor, is a protein that in humans is encoded by the RRH gene. Other proteins called opsins Photoreceptors can be classified several ways, including function (vision, phototaxis, photoperiodism, etc.), type of chromophore (retinal, flavine, bilin), molecular structure (tertiary, quaternary), signal output (phosphorylation, reduction, oxidation), etc. Beside animal opsins, which are G protein-coupled receptors, there is another group of photoreceptor proteins called opsins. These are the microbial opsin, they are useed by prokaryotes and by some algae (as a component of channelrhodopsins) and fungi, whereas animals use animal opsins, exclusively. No opsins have been found outside these groups (for instance in plants, or placozoans). Microbial and animal opsins are also called type