The targeting construct introduced a loxP site and an FRT-flanked neomycin resistance cassette into intron 21 and an F3 flanked puromycin cassette and loxP site into intron 22. Flp-mediated recombination removed the selection cassettes and left exon 22 flanked by loxP sites. This allele impacts the beta isoform and not the alpha isoform. (J:249969)