In addition to UCP1, expressed exclusively in brown adipose tissue (BAT), another member of the mitochondrial anion carrier protein family i.e. uncoupling protein 3 (UCP3) could play a physiological role in energy homeostasis. It is expressed almost exclusively in skeletal muscle and exhibits two transcriptional isoforms: a long form (UCP3L) and a short form (UCP3S). A brief caloric restriction resulted in ~2- to 3-fold increase in UCP3 mRNA levels in lean and obese humans.