SMPD1 and acid sphingomyelinase deficiency: Acid sphingomyelinase deficiency (ASMD) (historically Niemann–Pick disease [NPD] types A [OMIM257200], B [OMIM607616], A/B) results from pathogenic sequence variants in the SMPD1 (EC3.1.4.12) gene encoding the lysosomal enzyme acid sphingomyelinase (ASM).