ABCA1 (ATP-binding cassette, subfamily A member 1; also cholesterol efflux regulatory protein) is a 240-280 kDa member of the ABCA family, ABC transporter superfamily of molecules. Widely expressed, particularly by macrophages, ABCA1 participates in the transport of lipid across cell membranes. Once transported, ABCA1 is passed on to tethered ApoA-I to form HDL. Human ABCA1 is 2261 amino acids (aa) in length. It has up to 15 transmembrane segments with beginning and ending cytoplasmic domains and a large cytoplasmic central region. ABC transporters are found between aa 899-1311 and 1912-2144. Amino acids 2216-2221 are essential for lipid transport, and two intramolecular disulfide bonds are required for activity (Cys75-Cys309 and Cys1463/1465-Cys1477). There are three potential isoform variants of ABCA1. One possesses an alternative start site at Met61, while two others contain 12 aa and 123 aa substitutions for aa 241-2261, respectively. Over aa 1248-1350, human ABCA1 shares 97% aa identity with mouse ABCA1.