Human apo opsin proteins covalently bind the chromophore 11-cis-retinal (11cRAL) via a Schiff base linkage to a lysine residue in the seventh transmembrane alpha helix that is conserved in all known opsins (see review Shichida & Matsuyama 2009). The Schiff base linkage effectively results in an 11-cis-retinyl (11c-retinyl) group covalently linking to a lysine residue of opsins with subsequent loss of water. The three human cone opsins are Long Wavelength Sensitive Opsin (OPN1LW), Short Wavelength Sensitive Opsin (OPN1SW) and Middle Wavelength Sensitive Opsin (OPN1MW), sensing red, blue and green regions of the light spectrum, respectively (Nathans et al. 1986, Nathans et al. 1986b, Oprian et al. 1991).