There is heterogeneity in the phenotype of ADA-deficiency, with approximately 15–20% of patients exhibiting a ‘delayed clinical onset’ presenting with less severe, but gradually worsening, combined immune deficiency later in life, usually within the first decade, but occasionally in adulthood [8, 30, 31]. The gene discussed is ADA; the disease is hyperinsulinemic hypoglycemia, familial, 4.