GBA1 and Parkinson disease: 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).