Enzymology. A subfamily of mitogen-activated protein kinases that plays an important role in signal transduction and regulates cell proliferation, cell differentiation, and apoptosis. They activate transcription factor AP-1 via the phosphorylation of c-Jun proteins. They are responsive to stress stimuli, such as cytokines, ultraviolet irradiation, heat shock, and osmotic shock. They belong together with p38-MAPKs to the group of stress-activated protein kinases.