An arginine to glutamic acid (R120E) mutation was inserted in exon 3 and a floxed neo cassette was inserted in intron 4 via homologous recombination. Cre-mediated recombination removed the neo cassette. The exon 3 mutation changes a conserved positively charged amino acid in the first ligand binding domain to a negatively charged amino acid. The mutant protein shows surface expression in B-cells similar to wild-type protein but fails to bind ligand. (J:265415)