Acid sphingomyelinase deficiency (ASMD), also known as Niemann–Pick disease (NPD) types A or B, is an extremely rare genetic disorder characterised by mutations in the SMPD1 (sphingomyelin phosphodiesterase 1) gene, leading to a deficiency of the enzyme acid sphingomyelinase (ASM). The gene discussed is SMPD1; the disease is Niemann-Pick disease.