Moreover, in the postmortem brain tissue of PD patients, BiP levels significantly reduced in the temporal cortex and cingulate gyrus, but not in the caudate, prefrontal, or parietal cortex regions (Baek et al., 2019), demonstrating that the ER’s protein folding function is decreased in certain brain regions of PD patients. The gene discussed is HSPA5; the disease is Parkinson disease.