The amino acid sequence of the dual specificity mitogen-activated protein kinase kinase (MAPKK) has been determined by cDNA cloning and amino acid sequencing. MAPKK (393 residues, Mr 43,330) is a new member of the protein kinase subclass that comprises byr1 and STE7 that are involved in pheromone dependent signal transduction in yeast, wis1 a mitotic regulator in S. pombe and PBS2, which confers antibiotic resistance in S. cerevisiae.