IRE1alpha kinase-IN-1 is a potent and selective inhibitor of IRE1alpha (ERN1), with an IC50 of 77 nM for kinase and 80 nM for RNase activity, showing 100-fold selectivity over IRE1beta. It blocks ER stress-induced oligomerization and autophosphorylation, making it a valuable tool for studying the unfolded protein response in cellular and animal models of disease.