Low-density lipoprotein receptor 9 (LDL receptor) is a single-pass type I membrane protein which belongs to the LDLR family. It contains 3 EGF-like domains, 7 LDL-receptor class A domains, and 6 LDL-receptor class B repeats. This protein binds LDL, the major cholesterol-carrying lipoprotein of plasma, and transports it into cells by endocytosis. In order to be internalized, the receptor-ligand complexes must first cluster into clathrin-coated pits. In case of HIV-1 infection, it functions as a receptor for extracellular Tat in neurons, mediating its internalization in uninfected cells. Defects in LDLR will result in familial hypercholesterolemia.