Whilst the oxidants produced by MPO play an important role in the destruction of microorganisms and the inactivation of viruses, there is growing evidence that the increased activity of this enzyme and the accumulation of peroxidase-induced oxidative damage contribute to the development of many life-threatening diseases, many of which are related to the circulatory system, such as atherosclerosis or acute coronary syndrome. The gene discussed is MPO; the disease is atherosclerosis.