XBP1 and neoplasm: Since HO-1 is known to be elevated under hypoxia43, and both HO-1 and XBP1 have been shown to regulate hypoxia-inducible factor 1-alpha (HIF-1α) in hypoxic tumor cells44,45, we next examined the levels of a direct HIF-1α target, carbonic anhydrase IX (CAIX), in response to HO-1 overexpression.