MAPK3 and acute lymphoblastic leukemia: To further assess and confirm the role of Ca2+ on MEK/ERK activation under conditions of pevonedistat-induced ER stress/UPR leading to CHOP activation, we cultured ALL cells in Ca2+ free RPMI-1640 medium and examined the expression of p-ERK1/2, p-PKCα/β II, and p-CaMKII following the addition of Ca2+ and/or pevonedistat.