A neomycin resistance cassette flanked by loxP sites (floxed) replaced exon 3 of the gene. Mice carrying this allele were bred to mice that constitutively expressed cre-recombinase in all cells in early development (Schwenk et al., Nucleic Acids Res, 1995, 23:5080-5081). Resulting mice carrying the desired deletion were intercrossed to yield homozygous mutant mice at the desired locus. Northern blots of thymocytes and Con-A stimulated splenocytes from homozygous mutant mice showed no dectectable mRNA for the targeted gene. (J:42508)