The uncoupling proteins (UCPs) are found in the inner mitochondrial membranes and belong to a family of mitochondrial anion transport proteins that include the phosphate and oxologutarate carriers and the ADP/ATP translocator. Three UCP subtypes have been identified and cloned. UCP 1 is abundantly expressed in brown adipose tissue of mammals. UCP 1 allows proton re-entry into the mitochondrial matrix without involving the F0-F1 ATPase which produces heat instead of ATP. It is thought that UCP 2 and 3 also dissipate the mitochondrial proton gradient produced by the respiratory chain in cells and tissues within which they are expressed. Studies have shown UCP 2 to be expressed by a variety of tissues. UCP 3 is expressed predominantly in brown adipose tissue, skeletal muscle, at lower levels in heart and smooth muscle, and is absent in liver and kidney.