ABCA4 (ATP-binding cassette, sub-family A, member 4, ABCR) is a membrane-associated protein that functions to transport various molecules across the photoreceptor cell membrane in retina photoreceptor cells. ABCA4 is retina-specific, with N-retinylidene-PE as a substrate. Mutations in this gene are found in patients diagnosed with Stargardt disease, a form of juvenile-onset macular degeneration. Mutations are also associated with retinitis pigmentosa-19, cone-rod dystrophy type 3, early-onset severe retinal dystrophy, fundus flavimaculatus, and macular degeneration age-related 2. In immunohistochemistry of normal tissue, ABCA4 has membranous positivity in photoreceptor cells in the retina. It also has some positivity in a few regions of the brain (Warren, 2009). References: The UniProt Consortium. Nucleic Acids Res. 47: D506-515 (2019); Nucleic Acids Res. 2016 Jan 4;44(D1):D733-45, PMID:26553804; The Journal of Biological Chemistry. 2008. 283 (39): 26684–93, PMID: 18658157; Warren et al. Pharmacol Res. 2009 Jun;59(6):404-13, PMID: 19429473