The renin-angiotensin system (RAS) is highly conserved through phylogeny and is an essential regulator of blood pressure and water and electrolyte balance. This biological system is a multi-enzymatic cascade in which angiotensinogen (AGT), the substrate, is processed in a two-step reaction by the aspartyl protease renin and angiotensin-converting enzyme (ACE), resulting in the production of the decapeptide angiotensin-1 (Ang I) and the physiological active octapeptide angiotensin II (Ang II). In most species, renin is encoded by a single gene, whereas wild mice and certain strains of laboratory mice (in particular Sv129 from which are derived most embryonic stem, ES, cells) possess a second renin gene, which apparently arose from gene duplication [1, 2]. The two mouse genes, designated ren-1 and ren-2, encode highly homologous protein but the pattern of distribution and regulation of their expression differ substantially. The receptors for Ang II belong to the large family of G protein-coupled receptors. Based on pharmacological studies, two classes of receptors have been identified [3]. Studies using various non-peptide antagonists suggest that most of the classical effects of Ang II are mediated by AT1 receptors. Upon binding to AT1 receptors, Ang II induces a number of responses including increasing vascular smooth muscle tone, modulating tubular glomerular feedback, release of aldosterone from the adrenal glomerulosa, increasing renal sodium reabsorption, stimulation of thirst. Two subtypes of AT1 receptors, AT1A and AT1B have been cloned in rats and mice. However, only a single AT1 isoform has been identified in humans.