While intracellular (stored in granules of neutrophils and in inflammatory monocytes) and extracellular MPO (secreted and/or in neutrophil extracellular traps [NETs]) are implicated in host defense against microbial infections [36], [37]; in non-infectious diseases, it is thought that extracellular MPO contributes to the majority of unwanted oxidative stress and tissue damage [38], [39]. This evidence concerns the gene MPO and infectious disease.