c-Jun N-terminal kinases (JNKs) are MAP kinase family members that become highly activated after cells are exposed to stress conditions and are poorly activated by exposure to growth factors or mitogens. JNK inhibitor XVI is a selective, irreversible JNK inhibitor (IC50s = 4.67, 18.7, and 0.98 nM for JNK1, 2, and 3, respectively) that prevents phosphorylation of c-Jun in A375 and HeLa cells with EC50 values of 338 and 486 nM, respectively. It has been shown to inhibit JNK kinase activity by a mechanism that depends on covalent modification of cysteine116 in the ATP-binding motif. This compound has been used to explore the role of JNK in mediating cancer cell death.