The ABC transporter family has 48 members, classified into seven groups ABC A through ABC G based on sequence similarities, and involved in efflux of xenobiotics as well as biological macromolecules like peptides and lipids from the cytoplasmic compartment of the mammalian cell 134 136 viagra company Silverstein MJ, Barth A, Poller DN, Gierson ED, Colburn WJ, Waisman JR, et al