P4HB and Parkinson disease: Post-translational modifications of PDI such as S-nitrosylation (SNO-PDI) also accentuate the misfolding of synphilin in Parkinson's disease (Forrester et al., 2006), and SNO-PDI increases mutant SOD1 misfolding via incorrect disulfide cross-linking, leading to neuronal apoptosis (Jeon et al., 2014).