We have introduced a Ren-ID targeting vector Into embryonic stem cells containing the two highly homologous mouse renin genes Ren-ID and Ren-2. Using a polymerase chain reaction (PCR) screen designed to detect targeted integration at Ren-ID and Ren-2, we isolated 15 targeted embryonic stem cell clones, all of which had undergone a gene conversion event at the Ren-iD locus. We did not isolate any clones in which the incoming DNA had recombined with Ren-2. Over the region encompassed by our transgene, Ren-ID and Ren-2 display >95% homology. Our results suggest that the machinery driving gene targeting by means of homologous recombination in mammalian cells is capable of dis hing between these two sequences. Construction of transgenic mice with the embryonic stem cells reported here carrying a mutated renin gene will permit a greater understanding of the functions of the Ren-iD and Ren-2 gene products and their relative contribution to cardiovascular homeostasis.Renin is an aspartyl protease that catalyzes the initial and rate-limiting step in the conversion of angiotensinogen to the potent vasoactive hormone angiotensin II. The primary site of synthesis ofcirculating renin is thejuxtaglomerular cells of the kidney. However, a number of other extrarenal sites of renin biosynthesis have now been described, including the adrenal gland, liver, and testes (1-3). The function of this extrarenal renin is not known.In mice, high levels of renin and renin mRNA can also be found in the submandibular gland (4, 5). Mice are polymorphic for the number of renin genes, certain inbred strains harboring one gene (Ren-JC) and others containing two genes (Ren-JD and Ren-2). The mouse renin genes are located on chromosome 1 (6), with the Ren-2 gene situated -20 kilobases (kb) upstream ofRen-JD (relative to the transcriptional direction) in two-gene animals; both genes are transcribed in the same direction (7). The presence of the Ren-2 gene appears to be the result of a duplication of the renin locus t3-10 million years ago (8, 9). DNA sequence features unique to Ren-2 include the presence of a proviral intracisternal A particle located downstream and a B2 repetitive element situated upstream of Ren-2 (10, 11). However, Ren-JD and Ren-2 display a high degree of homology across the structural region of the gene having the same genomic organization (9,12,13) and sharing >96% sequence identity at the level of the transcript (9). At the genomic level (including intronic sequences) Ren-JD and Ren-2 share -95% sequence identity (ref. 13; unpublished results). The three mouse renin genes display similar but distinct tissue specificities (1,3,14,15) (Fig. 1) gg of linearized pTk-Ren-neo at 500 OLF and 250 V in a 0.4-cm pathlength electroporation chamber (Bio-Rad). These cells were then plated out on ten 9-cm Petri dishes. Selection with G418 (250 Ag/ml) and gancyclovir (2 puM) was applied after Abbreviation: ES, embryonic stem. tTo whom reprint requests should be sent at present address: