Protein levels of another UPR mediator, protein disulfide isomerase (PDI), did not show significant changes between the cell lines under basal conditions; however, PDI levels were significantly up‐regulated in N370S and L444P homozygous lines and N370S GBA1 mutant PD lines compared with controls upon MG132 treatment (Figure 4a,c). This evidence concerns the gene P4HB and Parkinson disease.