A striking example of pleiotropy is due to C9orf72 hexanucleotide repeat expansion mutation, which is clearly linked to ALS and FTD but pathogenic expansions have been also observed in a small percentage of patients affected by Alzheimer’s (<1%), Huntington’s (1–5%), and Parkinson’s diseases (1%), as well as atypical parkinsonian syndromes, such as progressive supranuclear palsy (1–8%), corticobasal degeneration (3%), and Lewy body dementia (2%) (van Blitterswijk et al., 2014b; Al-Chalabi et al., 2017; Balendra and Isaacs, 2018; Bourinaris and Houlden, 2018; Foxe et al., 2018). This evidence concerns the gene C9orf72 and Parkinson disease.