White and brown adipose tissues (WAT and BAT, respectively) play a central role in body weight and energy expenditure. WAT is the major site for energy storage via triglyceride synthesis and mobilization via lipolysis. Uncoupling proteins (UCP1-5) are a family of mitochondria transprt proteins that play a critical role in thermoregulatory heat production and maintenance of basal metabolic rate. BAT is able to dissipate energy as heat via uncoupled mitochondrial respiration by a mitochondrial anion carrier, uncoupling protein 1 (UCP1). UCP1 is predicted to contain 6 transmembrane (TM) domains, a putative purine nucleotide-binding domain (PNBD) and three-mitochondrial energy transfer protein domains (ETPDs). Both amino and C-termini are predicted to be cytoplasmic. Mouse/rat UCP2 is a 309aa (human 309aa, chromosome 7; ~95% homology) mitochondrial uncoupling protein. It is only 59% homologous with UCP1 found in brown adipose tissues. UCP2 has wide tissue distribution in mouse tissues (brain, kidney, liver, brown adipose tissue, heart and muscle). UCP2 may play a critical role in energy balance, body weight and thermoregulation.