For instance, several studies have found that decreased levels of amyloid β peptide of 42 amino acids length (Aβ42) in AD and of α-synuclein (α-syn) in PD in cerebrospinal fluid (CSF) indicate an overt development of the disease, but low specificity and contradictory results have been reported (10, 11). Here, SNCA is linked to Parkinson disease.