Strangely, Niemann-Pick type B patients with bi-allelic SMPD1 loss-of-function mutations, resulting in approximately 10% residual ASM activity [351], present rarely neurological symptoms, while a single heterozygous SMPD1 loss-of-function mutation, anticipating 50% residual ASM activity, increases the risk for PD. The gene discussed is SMPD1; the disease is Parkinson disease.