An FRT-flanked neomyocin resistance cassette with a 5' loxP site was inserted usptream of exon 4. An additional loxP site was inserted downstream of exon 7. Cre-mediated recombination removed exons 4 through 7 and the neomycin resistance cassette. RT-PCR confirmed the absence of transcript expresion in the spleen. (J:196483)