The intercellular transport of lipids through the aqueous circulatory system requires the packaging of these hydrophobic molecules into water-soluble carriers, called lipoproteins, and the regulated targeting of these lipoproteins to appropriate tissues by receptor-mediated endocytic pathways. The major classes of mammalian lipoproteins include low-density lipoprotein (LDL, the principal cholesteryl-ester transporter in human plasma), intermediateн density lipoprotein (IDL), high-density lipoprotein (HDL), very low-density lipoprotein (VLDL, a triglyceride-rich carrier synthesized by the liver), and chylomicrons (dietary triglyceride-rich carriers synthesized in the intestines).