Pro-form of Major Basic Protein 1 (ProMBP1) is composed of a strongly acidic 90-residue propart and a highly basic mature portion of 117 residues. It is an extensively and very heterogeneously glycosylated proteoglycan. ProMBP1 is processed to 14 kD MBP1 by convertase during maturation and localized in cell granule core. The mature MBP1 is a cytotoxic protein. The mAb J175-7D4 binds ProMBP1 and intermediate ProMBP1, but not mature MBP1. It specifically stain basophils, does not stain eosinophils, granulocytes, and mast cells and can be used as a marker for human basophils.