Targeted disruption precluded the expression of 3 isoforms (Ae2a, Ae2b1, and Ae2b2) produced by the wild-type allele. A 1.5 kb genomic fragment encompassing exons 2, 1b2, and 1b1 was deleted in the germline using a cre-loxP system. The deleted exons each served as the initial coding exons for the Ae2a, Ae2b1, and Ae2b2 isoforms. The absence of transcript encoding these isoforms was verifed by RT-PCR analysis of homozygous mutant testes RNA.