Achromatopsia (ACHM) is the inability to see color. Although the term may refer to acquire disorders such as color agnosia and cerebral achromatopsia, it typically refers to an autosomal recessive congenital color vision disorder, also called rod monochromacy and total congenital color blindness. Individuals with the congenital form of this disorder show complete absence of cone cell activity via electroretinography. There are at least four genetic causes of congenital ACHM, two of which are cyclic nucleotide-gated ion channels (ACHM2/ACHM3), a third the cone photoreceptor transducin (GNAT2, ACHM4), and the last unknown.