Based on a low neutrophil signal in a Phagoburst analysis, MPO deficiency was indicated, and diagnosis was genetically confirmed, showing as a homozygous deletion of 14bp in exon 9 of the MPO-gene (c.1555-1568del14 (ATGGAACCCAACCC) p.M519PfsX21, rs536522394), a previously described mutation in individuals with total MPO deficiency (18, 24, 38–40). This evidence concerns the gene MPO and hyperinsulinemic hypoglycemia, familial, 4.