A loxP site was inserted upstream of exon 2. An FRT-flanked neomycin resistance cassette with a 3' loxP site was inserted downstream of exon 3. Flp-mediated recombination removed the neomycin resistance cassette and left exons 2 and 3 floxed. Cre-mediated recombination removed exons 2 and 3. FACS analysis confirmed the absence of protein expression in cells from fetal day 17 spleen and thymus and adult bone marrow and spleen. (J:201231)