LoxP sites and an FRT-flanked neomycin selection cassette were inserted to flank exons 2 and 3. Flp-mediated recombination removed the selection cassette. Cre-mediated recombination in the germline resulted in the deletion of exons 2 and 3 and the neomycin selection cassette. Northern analysis confirmed the absence of mRNA expression in adult homozygous mutant kidneys.