A 2.3 kb genomic fragment containing the S1 and EC1 exons was replaced by a neomycin resistance cassette. Encoded transcript was undetectable in homozygous mutant mice via Northern blot analysis (data not shown). Western blot analysis of LPS-stimulated B cells derived from homozygous mice confirmed that no protein was made from this allele, and FACS analysis demonstrated that no functional protein was expressed on the cell surface of B cells in homozygous mice. (J:31133)