Myeloperoxidase (MPO) is a peroxidase enzyme that is highly expressed in neutrophil granulocytes. MPO produces hypohalous acids for antimicrobial processes and is the source of green coloration in pus and mucus. MPO may be a useful marker for cardiovascular diseases such as atherosclerosis, where increased concentrations of the protein have been linked to progression. In immunohistochemistry of normal tissue, it has positive cytoplasmic staining in neutrophils, Kupffer cells of the liver, and granulocytes such as eosinophils. In cancer, it is positive in myeloid leukemias and granulocytic sarcomas. References: Journal of Leukocyte Biology. 2005 May. 77 (5): 598–625, PMID 15689384; Clin Chem. 2009 Aug;55(8):1462-70, PMID: 19556446