ABCA1 (ATP-binding cassette, sub-family A (ABC1), member 1), also known as ABC1, the cholesterol efflux regulatory protein (CERP), is a protein which in humans is encoded by the ABCA1 gene. The membrane-associated protein encoded by this gene is a member of the superfamily of ATP-binding cassette (ABC) transporters. ABC proteins transport various molecules across extra- and intracellular membranes,contains 2,261 amino acids. Dot blot analysis of 50 tissues revealed ubiquitous expression of ABCA1 mRNA, with highest expression in placenta, liver, lung, adrenal glands, and all fetal tissues examined, and lowest expression in kidney, pancreas, pituitary, mammary gland, and bone marrow. This protein is a member of the ABCA subfamily. Members of the ABCA subfamily comprise the only major ABC subfamily found exclusively in multicellular eukaryotes. With cholesterol as its substrate, this protein functions as a cholesterolefflux pump in the cellular lipid removal pathway. Using human ABCA1 expressed in the membrane fraction of sf9 insect cells, Szakacs et al. found specific, Mg(2+)-dependent ATP binding and low basal ATPase activity. Addition of potential lipid substrates or lipid acceptors did not modify the ATPase activity or nucleotide occlusion by ABCA1. Szakacs et al. speculated that ABCA1 may be a regulatory protein or may require other protein partners for full activation.