The transbilayer distribution of phospholipids across the erythrocyte membrane is asymmetric; the amine-containing phospholipids, phosphatidylserine (PS) and phosphatidylethanolamine (PE), are enriched on the cytoplasmic surface of the membrane, whereas the choline-containing lipids, phosphatidylcholine and sphingomyelin, are enriched on the external surface (1, 2, 3, 4). In contrast to other phospholipids, the distribution of PS is absolute and is restricted entirely to the inner monolayer. This arrangement of lipids is generated by asymmetric biosynthesis and is maintained by a combination of passive (slow spontaneous flip-flop) and active transport processes. The distribution of phosphatidylserine is preserved by vanadate-sensitive, adenosine triphosphate (ATP)-dependent transport from the outer-to-inner monolayer. The protein responsible for this activity has been coined the aminophospholipid “flippase.” It is highly selective for phosphatidylserine and displays a strict requirement for the structure of its preferred substrate. Although the flippase transports N-methyl-PS, additional modification or deletion of the amine or carboxyl groups significantly reduces substrate transport (5,6). Flippase activity is absolutely dependent on the stereoconfiguration of the sn-2 position of the glycerol backbone, but not of the serine amino acid headgroup (5, 6, 7, 8). In addition to these substrate requirements, flippase activity is also sensitive to sulfhydryl reagents and Ca2+ (9, 10, 11).