A Frt-flanked neomycin cassette and a loxP site were placed downstream of exon 3. An additional loxP site was placed upstream of exon 3. Exon 3 and the neomycin cassette were deleted from the germline by crossing founder mice with cre-transgenic mice. Protein was undetectable in homozygote splenic lymphocytes as determined by immunoblot analysis. (J:151605)