A targeting vector introduced a cysteine (TGC) to alanine (GCG) substitution at amino acid position 373 (C373A) encoded within exon 8. A loxP flanked pGKNeo cassette was removed by crossing mice with Cre-transgenic mice, leaving behind a single loxP site. Western analysis of bone marrow derived mast cells from homozygotes indicated protein expression was similar to wild-type protein.