SMPD1 and Niemann-Pick disease: Niemann-Pick disease (NPD) is a rare inherited autosomal recessive lysosomal storage disorder (incidence about 1:40,000 in general population) caused by pathogenic mutations in theSMPD1 gene and characterised by enzyme studies showing a corresponding very low level of enzymatic activity of acidic sphingomyelinase (ASM), associated with intracellular accumulation of lipids1–3.