EIF2A and plasma cell myeloma: Analysis of a panel of human myeloma cell lines for several markers of ER stress and the UPR, including the ER chaperone BiP, XBP1s and phosphorylated eIF2α (p-eIF2α), showed marked variation in their basal expression (Figure 5A), implying differential sensitivities to proteasome inhibitors between myeloma cells.