A cell must selectively translocate molecules across its plasma membrane to maintain the chemical composition of its cytoplasm distinct from that of the surrounding milieu. The most intriguing and, arguably, the most important membrane proteins for this purpose are the ABC (ATP-Binding Cassette) transporters. These proteins, found in all species, use the energy of ATP hydrolysis to translocate specific substrates across cellular membranes. The chemical nature of the substrates handled by ABC transporters is extremely diverse--fr...