LoxP sites were inserted to flank exon 1. An frt-flanked neomycin selection cassette was also inserted 3' to exon 1. Cre-mediated recombination of the loxP sites resulted in a deletion of exon 1 and the neomycin selection cassette. Western blot analysis confirmed the absence of encoded protein in T cells from homozygous mutant mice. (J:259093)