Mutations in acid sphingomyelinase (aSMase), encoded by SMPD1, are causative of the NPD type A and B forms, whereas NPD type C, a lysosomal storage disease distinct from sphingolipidoses, is a cholesterol trafficking defect due to mutations in NPC1 or NPC2 genes [38]. The gene discussed is SMPD1; the disease is sphingolipidosis.