PTEN is a dual-specificity phosphatase at two levels. First, PTEN has been shown to dephosphorylate protein substrates on serine/threonine and tyrosine residues, thus acting as a dual-specificity protein phosphatase. One example is the tyrosine dephosphorylation of focal adhesion kinase （FAK） to inhibit cell spreading. Second, PTEN also dephosphorylates phosphatidylinositol 3,4,5-trisphosphate （PIP3） to phosphatidylinositol 4,5-bisphosphate （PIP2） — hence, PTEN is also a dual-specificity phosphatase in the sense that it dephosphorylates lipid substrates in addition to protein substrates.