Myeloperoxidase (MPO) is a peroxidase enzyme. MPO is most abundantly expressed in neutrophils, and produces hypohalous acids to carry out their antimicrobial activity. It is a lysosomal protein stored in azurophilic granules of the neutrophil and released into the extracellular space during degranulation. Neutrophil myeloperoxidase has a heme pigment, which causes its green color in secretions rich in neutrophils, such as mucus and sputum. Myeloperoxidase deficiency is a hereditary deficiency of the enzyme, which predisposes to immune deficiency. Antibodies against MPO have been implicated in various types of vasculitis, most prominently three clinically and pathologically recognized forms: granulomatosis with polyangiitis (GPA), microscopic polyangiitis (MPA) and eosinophilic granulomatosis with polyangiitis (EGPA). Recent studies have reported an association between elevated myeloperoxidase levels and the severity of coronary artery disease. MPO could serve as a sensitive predictor for myocardial infarction in patients presenting with chest pain. Immunohistochemical staining for myeloperoxidase used to be administered in the diagnosis of acute myeloid leukemia. Myeloperoxidase is the first human enzyme known to break down carbon nanotubes.