Regardless of the disease mechanism and the role for total MPO deficiency, it may be of great value to the clinician that encounters severe CNO or other inflammatory conditions with an unexpected severe phenotype to test for total MPO deficiency and to explore the clinical response to different cytokine blockers in a structured manner, especially TNFα blockers that was of such importance for this patient. The gene discussed is MPO; the disease is hyperinsulinemic hypoglycemia, familial, 4.