Mitogen-activated protein kinase kinase (also known as MAP2K, MEK, MAPKK) is a kinase enzyme which phosphorylates mitogen-activated protein kinase (MAPK). Mitogen-activated protein kinase kinase (also known as MAP2K, MEK, MAPKK) is a kinase enzyme which phosphorylates mitogen-activated protein kinase (MAPK). MAP2K is classified as EC 2.7.12.2.