A number of cellular processes, including cell proliferation and differentiation, appear to be regulated by the phosphorylation of proteins on tyrosine residues (1 ,2 ). The level of tyrosine phosphorylation of intracellular protein substrates is determined by the balance of phosphorylation by tyrosine kinase and dephosphorylation by phosphotyrosine phosphatase (PTPase) activities. Recent studies have proposed a role for PTPase in counterbalancing the growth-promoting effects of tyrosine kinases (3 -5 ). Because the enzymatic activity of PTPases far exceeds that of tyrosine kinases (6 -8 ), the PTPases may play an important physiological role in regulating growth, differentiation and neoplastic transformation.