ATP-binding cassette (ABC) proteins are one of the largest families of primary active transporters, which drive solute translocation by ATP binding and hydrolysis. The ABC transporter superfamily can be subdivided into import and export systems. ABC importers, which are involved in the uptake of amino acids, carbohydrates, peptides, vitamins, and metal ions, are exclusively present in Archaea and eubacteria. ABC exporters exist in all kingdoms of life and function as solute transporters, regulators, and ion channels with a broad solute specificity ranging from small inorganic ions to proteins of more than 100 kDa. In humans, ABC transporters are essential for various cellular processes, including lipid and sterol metabolism, iron homeostasis, detoxification and immune defense; they are therefore associated with a number of diseases, such as cystic fibrosis, hypercholesterolemia, subtypes of diabetes, adrenoleukodystrophy, macular degeneration, bare lymphocyte syndrome, and tumor development.