The monoclonal antibody NP4D6 specifically binds to CD203c (ENPP3), a type II transmembrane glycoprotein with enzymatic activity. CD203c is expressed on basophils and mast cells, and is markedly increased on activated basophils following allergen or IgE stimulation, making it a reliable marker of basophil activation in type I allergies. Its expression has also been observed in neoplastic cells in systemic mastocytosis, secretory endometrial glands and glioma cells. Flow cytometry with the NP4D6 clone is widely used in the diagnosis and monitoring of allergies, mast cell disorders and immunophenotyping studies.