Portions of exons 4 and 5 were replaced by a neomycin selection cassette inserted at the endogenous locus via homologous recombination. The deleted region encoded a portion of the extracellular domain which is common to both membrane bound and soluble forms of the protein. RT-PCR analysis of activated lymphocytes showed an absence of normal transcript in homozygous mutant mice. Cell surface expression of the encoded protien was undetected by flow cytometric analysis of cultured mast cells.