The ATP-binding cassette (ABC) superfamily is comprised of transmembrane proteins involved in energy-dependent transport of a variety of substrates across membranes. ABCA4 (also designated ABCR, photoreceptor RIM or RMP) is a photoreceptor specific ATP-binding cassette (ABC) transporter. ABCA4 is exclusively expressed within photoreceptor cells, indicating that ABCA4 mediates the transport of an essential molecule either into or out of photoreceptor cells. Mutations in the gene encoding ABCA4 are responsible for autosomal recessive Stargardt disease (STGD), an early onset macular degeneration, and some forms of autosomal recessive cone-rod dystrophy and autosomal recessive retinitis pigmentosa. In addition, heterozygosity for ABCA4 mutations may possess a risk factor for age-related macular degeneration. ABCA4 is most closely related to the mouse and human ABC1 and ABC2 and maps to human chromosome 1p22.1.