ABCA4 and retinitis pigmentosa: ABCA4 (ATP Binding Cassette Subfamily A Member 4) encodes an ATP-binding cassette (ABC) superfamily transmembrane protein in the retina photoreceptor cells, and mutations in this gene cause Stargardt Disease 1, Retinitis Pigmentosa 19, Cone-Rod Dystrophy 3 and Age-related macular degeneration.