A neomycin resistance cassette replaced 800 bp of the gene including part of exon 10, all of exon 11, and part of exon 12. The deleted sequences encode the carboxy-terminus, including the lectin homology domain with sequences that bind the C epsilon domain of IgE. Northern blot analysis of spleen did not detect transcript in homozygous mutant mice. Flow cytometric analysis did not detect protein on the surface of splenic B cells in homozygous mutant mice. Immunocytochemical analysis did not detect protein in asteroid dentritic cells near the corticomedullary junction of homozygous mutant thymus.