In a similar fashion to GBA1 mutations, homozygous mutations in SMPD1 (encoding acid sphingomyelinase or ASM) also cause an LSD, in this case, Niemann–Pick disease, whereas heterozygous SMPD1 variants are associated with increased risk of sporadic PD (Alcalay et al., 2019; Gan-Or et al., 2013; Foo et al., 2013; Mao et al., 2017; Dagan et al., 2015). Here, GBA1 is linked to Parkinson disease.