A neomycin selection cassette was inserted into exon 8. A 104 bp DNA fragment containing sequences encoding essential residues tyrosine 371 and histidine 374 was also deleted. Northern blot analysis on mRNA derived from LPS-induced macrophages from homozygous mice demonstrated that an aberrant 6.0 kb transcript is produced from this allele. However, western blot analysis confirmed that no detectable protein is produced from this allele in LPS-induced macrophages derived from homozygous mice.