Key evidence of the significance of OS in PD was (1) an increased ROS and heightened OS driven by endoplasmic reticulum (ER) stress, (2) alpha-synuclein (α-SN) accumulation that is affected by and contributes to OS, and (3) that extracellular DA itself promotes oxidation and increases the vulnerability to OS of DA-producing neurons in the SN, thus concluding that OS is the common denominator driving the development of PD [166]. The gene discussed is SNCA; the disease is Parkinson disease.