Exons 3 and 4 were deleted by the insertion of a floxed neo cassette, which was subsequently excised via the in vitro expression of cre recombinase. The targeted region encoded the transmembrane domain and a portion of the cytoplasmic domain including the first tyrosine of the immunoreceptor tyrosine-based activation motif (ITAM). The ablation of protein expression in homozygous mutant mice was confirmed via immunohistochemical analysis of spleen sections. (J:64763)