Mitogen-activated protein kinase (MAPK) cascades play essential roles in the signal transductions in eukaryotie. The MAPK cascades are composed of MAPK (mitogen-activated protein kinase), MAPKK (mitogen-activated protein kinase kinase) and MAPKKK (mitogen-activated protein kinase kinase kinase). These kinases are structurally conserved, and transduct signals via protein phosphorylation. Many kinases and genes of yeasts, animals and plants which are involved in the MAPK cascades have been isolated and characterized. From these investigations, the signal trasduction pathways of eukaryotic cells gradually become clear.