The Opsin in humans that is excited by the medium Wavelengths in the Visible Range (hence the “M” for medium/middle). The typical form has a Peak Wavelength at 540nm, which is a wavelength that excites green in Color Normals, hence the common colloquial name, Green Opsin (or Green Cone).
The opsin is encoded by the OPN1MW Gene on the X-Chromosome and is technically considered a type of LWS (Long Wave Sensitive) Opsin in the broad scale of vertebrate color vision. Indeed, the OPN1MW and OPN1LW genes are very similar and deviated very recently in human evolution. When the OPN1MW gene is missing (or altered enough to be inoperative), the individual exhibits Deuteranopia. When this gene has a minor mutation (usually a Hybridization with the OPN1LW that shifts the opsin to longer wavelengths, towards the L-opsin in the Absorption Spectrum), the opsin becomes the M*-Opsin, and the individual often exhibits Deuteranomaly.