SMPD1 and Parkinson disease: Moreover, sphingomyelin levels are significantly elevated in the substantia nigra of patients with PD (Riekkinen et al., 1975), and mutations in SMPD1/ASM are associated with an increased risk for developing PD (Foo et al., 2013; Gan-Or et al., 2013; Dagan et al., 2015).