The low density lipoprotein (LDL) receptor system coordinates the metabolism of cholesterol, an essential component of the plasma membrane of all mammalian cells. Study of this system has led to an enhanced understanding of the cellular basis of cholesterol homeostasis. It has also brought into focus an important mechanism of metabolic regulation--the process of receptor-mediated endocytosis. Data suggest that the juxtamembranous region of the cytoplasmic domain participates in protein:protein interactions that allow the low density lipoprotein receptor to cluster in coated pits. It has been shown that the family of LDL receptors may serve as viral receptors. Endocytosis of the Flaviviridae viruses, hepatitis C virus, GB virus C/hepatitis G virus, and bovine viral diarrheal virus (BVDV) was shown to be mediated by LDL receptors on cultured cells.