Publisher Summary Brown adipose tissue (BAT) is made of multilocular lipid-storing cells that are referred to as “brown adipocytes.” Biochemical studies have demonstrated that brown adipocyte mitochondria contain a unique membranous protein not found in any other type of cell. This protein, referred to as the “uncoupling protein (UCP),” allows brown adipocyte mitochondria to oxidize substrates rapidly without ADP phosphorylation, thus, promoting the dissipation of oxidation energy as heat. The function of BAT is particularly obvious in small mammals, which have an elevated metabolism. In large mammals, such as bovines, ovines, or humans, BAT is abundant at birth, but white adipose tissue rapidly becomes the dominant form of adipose tissue during development. Both white and brown adipocytes have a mesodermic origin. Although the existence of a common precursor, prior to brown and white adipose precursor cells, cannot be ruled out, it is believed that brown and white adipocytes derive from distinct fibroblastic precursor cells. To better understand the functional organization of the UCP, a recombinant expression of UCP or UCP mutants in yeast is a powerful system. The flow cytometry of yeast can be used to sort out cells according to their mitochondrial potential.