MEK1 (MAPKK1) catalyzes the concomitant phosphorylation of a threonine and a tyrosine residue in a Thr-Glu-Tyr sequence located in MAP kinases. MEKI (MAPKK1) activates ERK1 and ERK2 MAP kinases, enzymes which are involved in numerous signal transduction pathways. Defects in MEK1 (MAPKK1) are a cause of cardiofaciocutaneous syndrome (CFC syndrome), also known as cardio-facio-cutaneous syndrome. CFC syndrome is characterized by a distinctive facial appearance, heart defects and mental retardation. The sites on MEK 1 that are phosphorylated by MAP kinase are Thr292 and Thr386.