Dual specificity mitogen-activated protein kinase kinase 1 (MEK1/2) catalyzes the concomitant phosphorylation of a threonine and a tyrosine residue in a Thr-Glu-Tyr sequence located in MAP kinases. MEK1/2 activates ERK1 and ERK2 MAP kinases. Defects in MEK1/2 are a cause of cardiofaciocutaneous syndrome (CFC syndrome), also known as cardio-facio-cutaneous syndrome. CFC syndrome is characterized by distinctive facial appearance, heart defects and mental retardation.