A loxP site and FRT-flanked pgk-puro cassette were inserted upstream of exon 2 and a loxP site was inserted downstream of exon 4 via homologous recombination. Cre-mediated recombination removed the pgk-puro cassette and exons 2-4. RT-PCR and western blot analyses confirmed the absence of expression in testes from homozygous mice at 8 weeks of age. (J:251285)