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Figure 2 Efflux of cholesterol from peripheral cells. High-density lipoprotein (HDL) particles may accept free cholesterol (FC) from peripheral cells via several mechanisms. The ATP-binding cassette transporter A1 (ABCA1) effluxes FC to lipid-poor apolipoprotein (apo) A1 resulting in the formation of discoidal, nascent HDL. Esterification of the FC in nascent HDL by lecithin:cholesterol acyltransferase (LCAT) generates spherical, mature HDL particles. The lipidation of lipid-poor apo A1 and the conversion of discoidal HDL to spherical particles are rapid as evidenced by the very low levels of lipid-poor apo A1 and discoidal HDL in normal plasma. Free cholesterol may be effluxed to mature HDL particles by passive diffusion or by a receptor-mediated pathway, including the scavenger receptor B-1 (SR-B1) or the newly identified ATP-binding cassette transporter G1 (ABCG1). PL = phospholipid.