An intracellular signal molecule that forms part of a key signal transduction pathway in eukaryotic cells. It enters the nucleus to activate transcription factors that control gene expression. The so-called MAP kinase cascade is triggered by a small G protein, such as RAS protein, and consists of sequentially activating protein kinases, including MAP kinase kinase kinase (MAPKKK) and MAP kinase kinase (MAPKK); the latter doubly phosphorylates MAPK, enabling it to translocate to the nucleus and affect gene expression. This signal pathway is involved in diverse cellular activities and can be activated by a range of surface receptors.