A second screening was performed and 34 studies were discarded according to the following exclusion criteria: five were single nucleotide polymorphism (SNP) association studies with PD, another five were miRNA expression profile studies in PD patients, 12 studies did not investigate the impact on α-synuclein expression, two studies did not describe the impact of miRNAs expression, and 10 studies did not correlate miRNA and α-synuclein expression. Here, SNCA is linked to Parkinson disease.