BACKGROUND: Acid sphingomyelinase deficiency (ASMD) A and B, historically known as Niemann-Pick (NP) types A (NPA) and B (NPB), are life-threatening and rare inherited lysosomal storage disorders, caused by a deficiency in the acid sphingomyelinase enzyme activity. The gene discussed is SMPD1; the disease is acid sphingomyelinase deficiency.