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). This evidence concerns the gene SMPD1 and hereditary disease.