Exons 1 through 7 were replaced by homologous recombination of a cassette containing GFP and neo. Southern blot analysis confirmed that the targeting vector hybridized exclusively to Ren1 and not to Ren2. RT-PCR analyses of kidney tissue confirmed the absence of endogenous transcript in homozygous mutant mice. Expression of GFP was verified in both heterozygous and homozygous mutant mice. (J:72077)