Rhodopsin, the visual pigment of rod photoreceptors cells, is a member of the large family of G protein-coupled receptors. Rhodopsin is composed of two parts: a polypeptide chain called opsin and an 11-cis-retinal chromophore covalently bound to the protein by means of a protonated Schiff base linkage to Lys296 located in the seventh transmembrane segment of the protein. Several mutations have been described that constitutively activate the apoprotein opsin. These mutations appear to activate the protein by a common mechanism of action. They disrupt a salt-bridge between Lys296 and the couterion Glu113 that helps constrain the protein to an inactive conformation. Four of the mutations have been shown to cause two different diseases of the retina, retinitis pigmentosa and congenital night blindness. Recently, several other human diseases have been shown to be caused by constitutively activating mutations of G protein-coupled receptors.