The most frequent defects are the 3β-Δ5-hydroxy-C27-steroid oxidoreductase (3β-HSD) deficiency (OMIM 607765) which is due to mutations in HSD3B7; and to a lesser extent the Δ4–3-oxosteroid-5β-reductase (Δ4–3-oxo-R) deficiency (OMIM 235555) due to mutations in AKR1D1 [1–7]. The gene discussed is CHST3; the disease is hyperinsulinemic hypoglycemia, familial, 4.