SMPD1 and Niemann-Pick disease: Niemann–Pick disease (NP) is an LSD that is categorized into two variants: type A/B, which is caused by a mutation in the SMPD1 gene that leads to reduced activity of the enzyme acid sphingomyelinase (ASM), and type C, which is caused by mutations in the NPC1 and NPC2 genes that affect proteins involved in the transport of chol and other lipids.