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. Mice with this mutation exhibit a strong red fluorescent signal in the kidney, and E15.5 heterozygous mutant embryos exhibit fluorescence in the urogenital system that mimics the expression pattern of the endogenous gene.