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. PTEN protein, with the lipid and protein phosphatase activity, contributes to modulation of two main intracel- lular signal transduction pathways: growth factors/PI3K/Akt and Shc/Ras/Raf/MAPK, involved in regulation of cell growth, apoptosis, viability, adhesion and cell migration. PTEN is a crucial protein for normal cell development. Simultaneously, enzymatic activity of PTEN, its stability, subcellular localization and gene expression are under tight control. Studies on many cancer tissues imply that a decrease of biological activity of PTEN or complete lack of its function may result from alteration of PTEN gene expression due to mutation and/or epigenetic modification (mainly hypermethylation of promoter region of the gene). The explanation of the biological role of PTEN tumour suppressor protein and relationship between the protein activity and gene expression has great significance in searching for pharmacological strategies of inhibition of carcinogenesis at early steps of the process.