A loxP site was inserted upstream of exon 1. An FRT-flanked puromycin resistance cassette with a 3' loxP site was inserted downstream of exon 1. Flp-mediated recombination removed the selection cassette. Cre-mediated recombination removed exon 1. Western blot analysis confirmed the absence of protein expression in the spleen, kidney and peripheral lymph nodes. (J:231504)