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. The gene discussed is SNCA; the disease is Lewy body dementia.