IRE1α kinase-IN-1 is a highly selective IRE1α (ERN1) inhibitor, with an IC50 of 77 nM. IRE1α kinase-IN-1 displays 100-fold selectivity for IRE1α over the IRE1β isoform. IRE1α kinase-IN-1 inhibits ER stress-induced IRE1α oligomerization and autophosphorylation, and also inhibits IRE1α RNase activity (IC50=80 nM).