A targeting vector containing the Red Fluorescent Protein (RFP) gene followed, in reverse orientation, by two copies of a loxP and frt site-flanked kanamycin and neomycin resistance cassette (loxP-pA-frt-Kan&Neo-frt-loxP) was inserted into the first coding exon of the endogenous gene immediately following the ATG translation initiation codon. Red fluorescent signal is detected in the kidney and gonads of E15.5 heterozygous mutant embryos; fluorescence in the urogenital system mimics the expression pattern of the endogenous gene.