Our findings indicate that the loss of dopaminergic neurons would start a vicious cycle of decreased dopamine release—DRD2 signaling deficiency—decreased GSH synthesis—oxidative stress—dopaminergic neuron death during the progression of PD and explain the mechanism by which GSH decreases in the brains of PD patients (Supplementary Fig. 8o). The gene discussed is DRD2; the disease is Parkinson disease.