In both patients with Parkinson’s disease with diffuse Lewy bodies and those with Alzheimer’s disease, the brain tissue shows the S-nitrosylation of protein-disulfide isomerase (PDI), which was phenocopied in rotenone-treated SH-SY5Y cells, a cellular model of Parkinson’s disease. The gene discussed is P4HB; the disease is early-onset autosomal dominant Alzheimer disease.