Phosphatases are Enzymes that remove a phosphate group from its substrate. The addition of a phosphate group may activate or deactivate an enzyme or enable a protein-protein interaction to occur. Phosphatases are subdivided into four classes based on their substrate specificity: tyrosine-specific, serine-/threonine-specific, dual specific and histidine phosphatases. They often require a divalent metal ion for their activity. For example, the activity of serine/threonine phosphatases of the PPM family is Mg2+-dependent. Different phosphatases present distinct sensitivities to oxidation. Addition of Reducing Agents (e.g., GSH or DTT) is often required to maintain their activity, but there are some exceptions like shrimp alkaline phosphatase (SAP), which is inhibited by DTT.