MEK6 is a member of MAPKK protein kinase family. By using degenerate oligonucleotide primers from the conserved kinase domains of MKK3 and MKK4 two human cDNAs and 1 murine cDNA encoding closely related proteins of the MKK family were cloned. The two human clones appear to be different isoforms of the same gene generated by differential splicing: the shorter clone was designated MKK6, encodes a 278-amino acid protein, while the longer clone, designated MKK6b, encodes a 334-amino acid protein. MKK6 is about 80% identical to MKK3 and 40% identical to MKK4. 1.7-kb human MKK6 transcript is highly expressed in skeletal muscle, while an MKK6b-specific probe detected mRNA bands of 1.8, 2.4, and 4.5 kb that are enriched in heart, skeletal muscle, pancreas and liver. MKK6 plays an important role in intracellular signaling pathways leading toward activation of the p38 MAP kinase . MEK6 phosphorylates and activates p38 in response to inflammatory cytokines or environmental stress. As an essential component of p38 MAPK mediated signal transduction pathway, this gene is involved in many cellular processes such as stress induced cell cycle arrest, transcription activation and apoptosis.