Spread of α-synuclein could also possibly explain the Braak hypothesis that Lewy pathology in PD patients undergoes a predictable distribution pattern starting in the lower brainstem and olfactory bulbs in prodromal disease, progressing to the midbrain region at disease diagnosis, and eventually reaching cortical regions at later disease stages (Braak et al., 2003). This evidence concerns the gene SNCA and Parkinson disease.