MPO and hyperinsulinemic hypoglycemia, familial, 4: MPO deficiency is the most common inherited defect in phagocytes and it is associated with impaired microbial killing.1 The acquired form is much less frequent than the hereditary form and, as observed in our patient, it is usually transient and reverses after improvement of the underlying conditions.1 Moreover, the acquired form has been described to be partial and to involve only a fraction of the neutrophils.