The tumour suppressor protein PTEN (also called MMAC1 and TEP1) is a dual specificity phosphatase, recognizing both lipid and protein substrates. By its lipid phosphatase activity, PTEN dephosphorylates phosphatidylinositol 3,4,5-triphosphate (PIP-3), which leads to negative regulation of serine-threonine kinase Akt (PKB). By its protein phosphatase activity, PTEN participates in regula- tion of MAP kinase signalling pathway through dephosphorylation of FAK and Shc proteins.