XBP1 and melanoma: To determine whether or not XBP1 activation was PERK‐dependent or simply thapsigargin dependent, three different inhibitors of the PERK‐dependent ER‐stress response (Fig. S3A) were tested for their ability to prevent thapsigargin induction of ER‐stress response genes under conditions that did not inhibit melanoma A375 proliferation (Fig. S3B).