ATP-binding cassette (ABC) transporters 

ATP-binding cassette (ABC) transporters are a large group of membrane proteins, comprising 49 transporters that are further subdivided into seven subfamilies, ABC ­A to ­G, based on sequence similarities, that have two nucleotide-binding domains (NBDs) and two transmembrane binding domains (TMDs). ATP-binding cassette (ABC) transporters couple transport of a substrate against a chemical gradient and energized directly by the hydrolysis of ATP. ATP-binding cassette transporters transport a wide variety of substrates, including inorganic and organic small molecules like metal ions, sugars, amino acids, nucleotides, and vitamins, as well as larger organic compounds such as peptides, proteins, oligonucleotides, cellular metabolites, and drugs. ATP-binding cassette (ABC) transporters are involved in many important physiological processes like nutrient uptake, cellular detoxification, lipid homeostasis, signal transduction, viral defense, and antigen presentation. ATP-binding cassette (ABC) transporters play an important role in a number of diseases, including Stargardt disease, Alzheimer's disease, immune deficiency, and cystic fibrosis^[1][2].