A loxP site was inserted upstream of exon 17. An FRT-flanked neomycin resistance cassette with a 3' loxP site was inserted downstream of exon 17. Flp-mediated recombination removed the neomycin resistance cassette. Cre-mediated recombination removed exon 17. The absence of the long protein isoform expression was confirmed by western blot analysis on T cell extracts. (J:180016)