Blue cone monochromatism (BCM) is a recessive X-linked disease characterized by severely impaired color discrimination, low visual acuity, nystagmus, and photophobia, due to dysfunction of the red (L) and green (M) cone photoreceptors. BCM is as an incomplete form of achromatopsia (see this term). The disease is Orphanet rare-disease nomenclature entry Orphanet_16 (Blue cone monochromatism). Also known as: Atypical X-linked achromatopsia, Blue cone monochromacy, Color blindness, blue monocone monochromatic type, Colour blindness, blue monocone monochromatic type, S cone monochromacy, S cone monochromatism, X-linked incomplete achromatopsia.