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