Cholesterol is an essential component of all animal cell membranes, especially the limiting plasma membrane. It must be made available in sufficient amount either by synthesis in situ or by acquisition from the diet. Yet, the accumulation of excess cholesterol in cells and tissues can be toxic. Thus, a highly regulated system has evolved for the synthesis and distribution of cholesterol within and between cells. This is particularly important in the brain, which by far has the highest concentration of cholesterol in the body. Indeed, the central nervous system has about 25% of the total body cholesterol of an adult animal.