ABCA4 (ATP-Binding Cassette, Subfamily A, Member 4), also known as ABCR, is a protein which in humans is encoded by the ABCA4 gene. ABCA4 is a member of the ATP-binding cassette transporter gene sub-family A (ABC1) found exclusively in multicellular eukaryotes. Using a whole genome radiation hybrid panel, Allikmets et al. (1997) mapped the ABCR gene to 1p21-p13. Allikmets et al. (1997) localized ABCR transcripts exclusively within photoreceptor cells, indicating that ABCR mediates the transport of an essential molecule (or ion) either into or out of photoreceptor cells. Molday et al. (2000) showed by immunofluorescence microscopy and Western blot analysis that ABCR is present in foveal and peripheral cone, as well as rod, photoreceptors. The results suggested that the loss in central vision experienced by patients with Stargardt macular dystrophy arises directly from ABCR-mediated foveal cone degeneration.