CD243 (MDR-1) belongs to the ATP binding cassette (ABC) transporter family. With an approximate molecular mass of 170 kD, it consists of two homologous halves. Each half contains two hydrophobic transmembrane domains (TMDs) and two hydrophilic nucleotide binding domains (NBDs). The TMDs span the membrane six times, forming a chamber with a 12 transmembrane alpha-helix structure. NBDs drive the transport process through ATP coupling and hydrolysis and are located at the cytoplasmic face of the membrane. CD243 transports various molecules across cellular membranes and is involved in multidrug resistance. MDR-1 is expressed on hematopoietic stem cells, T cells, B cells, and NK cells as well as on many multidrug resistant neoplastic cells. CD243 interacts with Caveolin, RING finger protein 1B, AAP1, p53, Orphan nuclear receptor PX, and cytochrome P450.