A loxP site was inserted upstream of exon 21. An FRT-flanked neomycin resistance cassette with a 3' loxP site was inserted downstream of exon 22. Flp-mediated recombination removed the neomycin resistance cassette and left exons 21 and 22 floxed. Cre-mediated recombination removed exons 21 and 22. Western blot analysis confirmed the absence of protein expression in the testis.