An isoform-specific disruption was generated by inserting a cassette containing eGFP downstream of the start codon of exon 1gammat. The targeted exon is the third exon of the gene, serves as an alternative first exon for the gammat transcript, and is not included in the gamma transcript. RT-PCR analysis of hepatic RNA obtained from homozygous mutant mice identified gamma transcript but not the targted gammat transcript. The eGFP sequence included a stop codon but no splice sites or polyadenylation signals. A floxed neo cassette included in the targeting vector was excised from the germline via cre-mediated recombination.