In addition to the clinical overlap between dementia and parkinsonism in carriers of PSEN1 mutations, there is extensive Lewy body pathology in early-onset AD carriers of the PSEN1 p.S170F (Snider et al., 2005) and PSEN1 p.A431V mutations (Qiao et al., 2017) suggesting an interaction between PSEN1 dysfunction and α-synuclein aggregation. This evidence concerns the gene SNCA and Parkinson disease.