The third major discovery was the fact that 3–7% of patients with idiopathic PD carry a heterozygous mutation for the gene glucocerebrosidase A. Genome-wide association studies have confirmed—besides the role of alpha-synuclein—the importance of the microtubule-associated protein tau (MAPT) in the etiopathogenesis of PD. This evidence concerns the gene MAPT and Parkinson disease.