Aggregation of alpha‐synuclein, encoded by SNCA (OMIM#163890), also referred to as NACP (nonamyloid component of plaques), is a main component of Lewy bodies, the hallmark of Parkinson's disease (PD) and other Lewy body dementias, and is also implicated in cases of Alzheimer's disease. This evidence concerns the gene SNCA and early-onset autosomal dominant Alzheimer disease.