The main focus of transgenic animal models in the study of adipose tissue biology has been the expression or knockout of selected genes specifically in adipose tissue. Adipose-tissue-specific expression/knockout of genes became possible with the identification and characterization of promoter regions conferring adipose-tissue-specific expression. The promoters for the adipocyte lipid binding protein aP215 and for phosphoenolpyruvate carboxykinase (PEPCK)16 are commonly used to target both white and brown adipose tissue (WAT/BAT), whereas the UCP-1 promoter only targets BAT.17 Using the recombinase activity of the cyclization recombination (Cre) gene from the P1 bacteriophage, conditional transgenesis and knockout of specific genes became a commonly used method to create transgenic animal models. The Cre recombinase, a 38-kDa site-specific DNA recombinase, catalyses the recombination of two 34-bp-long loxP (locus of X-over of P1) sites, making it possible to excise loxP-flanked DNA segments.10 and 18 The major application of the Cre/LoxP system is the generation of spatially or temporally regulated gene knockouts.19 Cre/LoxP-mediated recombination requires two steps. First, the endogenous gene to be targeted is modified by homologous recombination in murine embryonic stem (ES) cells so that LoxP sites flank a portion of the gene, and mice are generated from these recombinant ES cells to obtain flox (flanked by lox) mice. Second, the heterozygous flox mice are bred with mice expressing a Cre transgene to generate double heterozygous mice. These double heterozygous mice are bred with single heterozygous flox mice to generate conditional knockout mice