A loxP site was inserted upstream of exon 4, an FRT site was inserted upstream of exon 4a, and a neo cassette flanked by attP and attB with 3' FRT and loxP site was inserted downstream of exon 4a. Flp mediated recombination removed the neo cassette and exon 4a. Exon 4 remains floxed. Real-time RT-PCR confirms the absence of the kidney specific transcript isoform.