APY29 is a highly potent and selective allosteric modulator of IRE1α. Under endoplasmic reticulum (ER) stress, unfolded proteins accumulate in the ER to activate the ER transmembrane kinase/endoribonuclease (RNase)—IRE1α. IRE1α oligomerizes, autophosphorylates, and initiates splicing of XBP1 mRNA, thus triggering the unfolded protein response (UPR). APY29 Inhibits IRE1α autophosphorylation (IC₅₀ = 280 nM) and activates IRE1αRNase activity. As dysregulation of the UPR has been implicated in a variety of cell degenerative and neoplastic disorders, small molecule modulators of IRE1α should be very useful in understanding UPR’s role in pathophysiology and to develop drugs for ER stress-related diseases.