Dual-speclflcity phosphatases, a subfamily enzymes of protein tyrosine phosphatases, play important roles in signal transduction and cell-cycle regulation during cell growth and differentiation (1 ). Unlike classic tyrosme phosphatases which are specific for phosphotyrosine, dual-specificity enzymes can dephosphorylate both phosphotyrosme and phosphoserine/phosphothreonine. All tyrosine phosphatases, including dual-specificity enzymes, are characterized by a conserved-sequence motif HCXXGXXRS/T (2 ). Besides this activesite region, dual-specificity phosphatases have little primary-sequence homology to classic tyrosine phosphatases, but they do bear remarkable similarity at the three-dimensional structure level, especially around the catalytic center (3 ,4 ). Dual-specificity enzymes appear to use the same catalytic mechanism as the classic tyrosine phosphatases, and catalysis is proceeded by formation of a cysteine-phosphate intermediate (5 ,6 ). Mutation of the essential cysteine at the active site in dual phosphatases abolishes the phosphatase activities toward both phosphotyrosine and phosphoserine/phosphothreonine.
