Aβ42, amyloid-beta 42; Aβ40, amyloid-beta 40; α-Syn, alpha-synuclein; AD, Alzheimer's disease; AP, atypical parkinsonism; APS, atypical parkinsonian syndromes; CSF, cerebrospinal fluid; DLB, dementia with Lewy bodies; iPSC, induced pluripotent stem cells; MDS, Movement Disorder Society; MS, multiple sclerosis; PD, Parkinson's disease; p-tau, phosphorylated tau; RBD, rapid eye movement sleep behavior disorder; SNCA, synuclein alpha gene; t-tau, total tau. The gene discussed is SNCA; the disease is early-onset autosomal dominant Alzheimer disease.