Tryptase (EC 3.4.21.59) is a serine proteinase found in mast cells and is commonly used as a biomarker for mast cell activation. It is thought to cleave the hemagglutinin surface protein of influenza A virus, resulting in its activation and consequential flu symptoms. Serum tryptase levels are also higher in eosinophilic leukemias with FIP1L1-PDGFRA fusion mutations and can be used in their identification. References: American Journal of Clinical Pathology. 144 (3): 377–92. doi:10.1309/AJCPMORR5Z2IKCEM. PMID 26276769; Biochimica et Biophysica Acta. 957 (1): 71–80. doi:10.1016/0167-4838(88)90158-6. PMID 3140898;