NPD-A and NPD-B are due to enzyme acid sphingomyelinase deficiency caused by SMPD-1 gene (sphingomyelin phosphodiesterase 1) missense mutation, and NPD-C is due to NPC-1 and NPC-2 (Niemann-Pick C1 protein and Niemann-Pick C2 protein) gene mutation. The gene discussed is SMPD1; the disease is hyperinsulinemic hypoglycemia, familial, 4.