A neomycin selection cassette was inserted into exon 2, creating a new stop codon 239 bp downstream of the integration site. RT-PCR analysis on RNA isolated from activated macrophages, mast cells and NK cells of homozygous mice demonstrated that no detectable transcript was produced from this allele. Western blot analysis on lysates derived from activated macrophages, mast cells and NK cells of homozygous mice confirmed that no protein is encoded by this allele.