c-Jun is a component of the transcription factor AP-1 that binds and activates transcription at TRE/AP-1 elements. The transcriptional activity of c-Jun is regulated by phosphorylation at Ser63 and Ser73. Extracellular signals, such as growth factors and UV irradiation, stimulate phosphorylation of c-Jun at Ser63/73 and activate c-Jun dependent transcription. The MAP kinase homologue, SAPK/JNK, binds to the N-terminal region of c-Jun and phosphorylates c-Jun at Ser63/73.