MAPK8(Mitogen-activated protein kinase 8) is also named as JNK1, PRKM8, SAPK1, SAPK1C and belongs to the MAP kinase subfamily. MAPK8 is activated by dual phosphorylation at a Thr-Pro-Tyr motif during response to UV light. MAPK8 functions to phosphorylate c-Jun at N-terminal serine regulatory sites of Ser-63 and Ser-73, mapping within the transactivation domain. Phosphorylation of these sites in response to UV results in transcriptional activation of c-Jun. It has some isoforms produced by alternative splicing with the molecular weight of 46 kDa and 48 kDa.