Genetics and pathology of Parkinson's disease (PD). 90% cases of PD are sporadic, whereas 10 % cases are hereditary. α-synuclein (SNCA) protein is shown to be a major component of Lewy body, a pathological hallmark of both sporadic and familial form of PD, suggesting that dysfunction or toxicity caused by SNCA protein results in the pathology of PD. Importantly, duplication and triplication of SNCA gene cause dominant form of PD, PARK4. Hence, decreasing levels of SNCA expression is thought to be an attractive treatment for suppressing progression of PD.