Obesity is viewed as an imbalance in the regulation of energy input and energy expenditure of an individual. The many components in energy homeostasis, i.e. food intake, physical activity, basal activity and heat production, make obesity a complex problem. We seek to understand the role of non-shivering thermogenesis, the process for heat production by brown fat, in this process. The capacity for non-shivering thermogenesis depends on activation of the mitochondrial uncoupling protein (UCP), lipolysis, and respiration by sympathetic innervation and the amount of brown fat in an individual. The first specific aim seeks to identify the cis-acting DNA sequences which restrict expression of the Ucp gene to brown fat only and stimulate its expression in response to cold. The primary focus in this specific aim is to identify sequences, which interact with a brown fat specific transcription factor, by a combination of transient expression, gel retardation and DNA footprinting analyses of nested deletions of the promoter region and DNAase 1 sensitive regions. The second specific aim seeks to clone the transcription factor from a lambda gt11 expression cDNA library. The cDNA clones will then be used in the third specific aim to determine whether a brown fat specific transcription factor can induce brown fat differentiation from a white fat cell lineage, by analogy with the role that a muscle transcription factor, Myo D, has in inducing muscle cell differentiation, The fourth specific aim will evaluate the in vivo function of the transcripton factor by inactivating the endogenous gene in embryonic stem cells by homologous recombination. Cells carrying the mutated gene will then be introduced into the animal by microinjection into blastocysts to obtain live offspring with a mutated gene.