Extracellular stimuli such as growth factors and UV irradiation stimulate phosphorylation of c-Jun at Ser63/73 and activate c-Jun dependent transcription. Mutation at Ser63/73 renders c-Jun nonresponsive to mitogenic and stress induced signaling pathways. JNK (c-Jun N-terminal Kinase) binds to the N-terminal region of c-Jun and phosphorylates c-Jun at Ser63/73. The activity of JNK is stimulated by the same signals that activate c-Jun.