Flow cytometry (FCM) immunophenotyping using a fluorescein‐conjugated anti‐MPO monoclonal antibody (right) on the Navios flow cytometer (Beckman Coulter) confirmed that neutrophils and monocytes from the patient had MPO deficiency (11% and 22% of the normal, respectively); in addition, part of the neutrophils (36%) was degranulated, as evaluated by the side scatter (SSC) and this abnormal neutrophil population had lower MPO levels than the remaining neutrophils, with an almost complete MPO deficiency (3% vs 16%, respectively). This evidence concerns the gene MPO and hyperinsulinemic hypoglycemia, familial, 4.