A family of proteins that span the plasma membranes of cells and function to transport specific molecules into or out of the cell. The name is an abbreviation of ATP-Binding Cassette. ABC transporters all contain an ATP binding domain, and they utilize the energy of ATP to pump substrates across the membrane against a concentration gradient. The substrates may be amino acids, sugars, polypeptides, or inorganic ions. The product of the cystic fibrosis gene is an ABC transporter. See Bacillus, cystic fibrosis (CF), Escherichia coli.