The reversible phosphorylation of structural and regulatory proteins in eucaryotic cells is one of the most important regulatory mechanisms. Protein tyrosine phosphatases (PTP) regulate a wide range of signal transduction pathways that control many cellular processes such as cell proliferation, differentiation and growth. Disorder in PTP gene expression is implicated in the development of cancer, autoimmune and neurodegenerative diseases. The active sites of these enzymes are characterized by the consensus sequence containing cysteine which is essential for enzyme activity and highly susceptible to oxidation. Reversible oxidation of the catalytic cysteine is becoming recognized as a general mechanism for regulation of PTP enzymatic activity. These findings suggest that protein tyrosine phosphatases may be considered as very sensitive markers of oxidative stress. Many studies have demonstrated that the production of reactive oxygen species during oxidative stress can inactivate protein tyrosine phosphatases.