Recently, IRE1α-mediated X-box–binding protein-1 (XBP1) signaling was reported to promote GVHD (9, 17), but the mechanism of how the ER stress sensors PERK and XBP1 regulate T cell allogeneic response and GVHD induction is essentially uncharacterized. The gene discussed is XBP1; the disease is graft versus host disease.