A cassette containing a lacZ gene followed by a floxed neo gene was inserted at the start codon in exon 1. RT-PCR and Northern blot analysis of splenic RNA showed an absence of transcript in homozygous mutant mice. Mice heterozygous for this allele were bred with transgenic mice bearing a constitutively expressed cre recombinase gene to generate F2rl3tm1.1Cgh mice.