Exons 3 through 7 and part of exon 8 (including the TGA stop codon) were replaced with a Neomycin resistance gene. These exons encode the second extracellular loop, the second through fourth membrane spanning domains and the entire C-terminal intracellular domain. Absence of gene expression was established by Northern blot analysis and RT-PCR and absence of protein was confirmed by cytofluorometric analysis of splenic B cells.