The majority of ATP-binding cassette (ABC) transporter family are primary active transporters that use ATP hydrolysis to drive the transport of substrates across the membrane. A functional transporter contains 12 transmembrane helices and 2 cytoplasmic nucleotide binding domains. The transporter may be encoded by one polypeptide (full transporters) or formed from by a dimer of half transporters. ABC transporters can be divided into 8 subfamilies, A-H, based on sequence similarity. Members of subfamilies E and F do not contain transmembrane domains and are not transporters. (Adapted from FBrf0224053).