A neomycin resistance gene replaced the first coding exon, which includes the first zinc finger of the DNA-binding domain, via homologous recombination. Northern blot and RT-PCR detected transcript ~200 bp shorter than the wild-type transcript, as expected. Sequence analysis revealed that exon 1 was fused to exon 3 in the mutant animal. The truncated transcript was present at reduced levels. If this RNA is translated from a spurious initiation site, the protein is expected to be nonfunctional.