Catalyzes the concomitant phosphorylation of a threonine and a tyrosine residue in a Thr-Glu-Tyr sequence located in MAP kinases. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Expressed abundantly in the adult brain and muscle. It is initially expressed in the dorsal region of the embryo in a diffuse pattern at stage 17. Subsequently expression occurs in the early stages of development of the central nervous system, including the brain, spinal cord and eye. Later (stage 24) expression is found in the hindbrain, midbrain and forebrain and the somites. By stage 32, the expression is detected in the cranial neurons. Expression in the brain is increased, while expression in the spinal cord has decreased by stage 37. MAPKK is itself dependent on Ser/Thr phosphorylation for activity catalyzed by MAP kinase kinase kinases. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase subfamily.