Cholesterol is a sterol molecule essential for maintenance of cell membranes, regulating membrane fluidity, and establishing membrane permeability. Cholesterol is also the molecular precursor for vitamin D and several sex steroid hormones (estrogen, testosterone, progesterone, cortisol, and aldosterone). Produced throughout the body and absorbed from the diet, physiological levels of cholesterol are dependent on the specific tissue of interest, with highest levels in liver, bone and spinal cord. Because cholesterol is insoluble in the blood, it is transported throughout the body via low-density (LDL) and high-density (HDL) lipoproteins. Once inside the cell, cholesterol is transported by fatty acid binding proteins (FABP). Cholesterol also acts as an important constituent of bile, released from the gallbladder, where it is oxidized into bile acids to enhance digestion and absorption of dietary fats. Much research into lipid/cholesterol metabolism focuses on the condition of hypercholesterolemia, where decreased HDL and increased LDL levels augment the degree of atherosclerosis and risk of cardiovascular disease.