OPN1MW2 is a duplication of the OPN1MW gene, which encodes the medium wavelength sensitive (MWS) photopsin. The gene duplication is present in about 50% of X-chromosomes, so is present in 50% of males and at least once in 75% of females. It caused by the same mechanism that causes congenital red-green color blindness, the most common form of color blindness.