to 50 for the most conserved residue in that transmembrane domain among GPCRs known in 1995. For instance in the seventh transmembrane domain, the proline in the highly conserved NPxxY7.53 motif is Pro7.50, the asparagine before is then Asp7.49, and the tyrosine three residues after is then Tyr7.53. Another numbering scheme is based on cattle rhodopsin. Cattle rhodopsin has 348 amino acids and is the first opsin whose amino acid sequence and whose 3D-structure were determined. The cattle rhodopsin numbering scheme is widespread in the opsin literature. Therefore, it is useful to use both schemes. The retinal binding lysine Opsins without the retinal binding lysine are not light sensitive. In cattle rhodopsin, this lysine is the 296th amino acid and thus according to both numbering schemes Lys2967.43. It is well conserved among opsins, so well conserved that sequences without it were not even considered opsins and thus excluded from large scale phylogenetic reconstructions. Even so most opsins have Lys2967.43, some have lost it during evolution: In the nemopsins from nematodes, Lys2967.43 is replaced by Arginine. In the astropsins from sea urchins and in the gluopsins, Lys2967.43 is replaced by glutamic acid. A nemopsin is expressed in chemosensory cells in Caenorhabditis elegans. Therefore, the nemopsins are thought to be chemoreceptors. The gluopsins are found in insects such as beetles, scorpionflies, dragonflies, and butterflies and moths including model organisms such as the silk moth and the tobacco hawk moth. However, the gluopsins have no known function. Such function does not need to be light detection, as some opsins are also involved in thermosensation, mechanoreception such as hearing detecting phospholipids, chemosensation, and other functions. In particular, the Drosophila rhabdomeric opsins (rhabopsins, r-opsins) Rh1, Rh4, and Rh7 function not only as photoreceptors, but also as chemoreceptors for aristolochic acid. These opsins still have Lys2967.43 like other opsins. However, if this lysine is replaced by an arginine in Rh1, then Rh1 loses light sensitivity but still responds to aristolochic acid. Thus, Lys2967.43 is not needed for Rh1 to function as chemoreceptor. Also the Drosophila rhabopsins Rh1 and Rh6 are involved in mechanoreception, again for mechanoreception Lys2967.43 is not needed, but needed for proper function in the photoreceptor cells. Beside these functions, an opsin without Lys2967.43, such as a gluopsin, could still be light sensitive, since in cattle rhodopsin, the retinal binding lysine can be shifted from position 296 to other positions, even into other transmembrane