Loss-of-function mutations in the sphingomyelin phosphodiesterase 1 (SMPD1) gene are associated with decreased catalytic activity of acid sphingomyelinase (ASM) and are the cause of the autosomal recessive lysosomal storage disorder Niemann-Pick disease (NPD) types A and B. Currently, >100 missense mutations in SMPD1 are listed in the Human Gene Mutation Database. Here, SMPD1 is linked to Niemann-Pick disease.