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PTEN encodes a major tumor-suppressor protein that is a dual-specificity phosphatase. Inactivation of PTEN has been shown to be involved in heritable and sporadic cancers. Mutation or deletion of PTEN, historically the most commonly identified mechanisms of inactivation of tumor suppressors, is found only in the minority of sporadic non-cultured primary cancers, which indicates that there might be other, novel mechanisms of inactivation. Despite the absence of a classic nuclear localization signal, PTEN enters the nucleus by several mechanisms, including simple diffusion, active shuttling, cytoplasmic-localization-signal-dependent export and monoubiquitylation-dependent import. Cytoplasmic PTEN has a well-known role as a negative regulator of the PI3K/AKT pathway; however, it is becoming clear that cytosolic PTEN is not the same as nuclear PTEN. Nuclear PTEN plays a role in chromosome stability, DNA repair, cell cycle arrest and cellular stability. The balance between these functions is an important factor in determining whether a cell remains benign or becomes neoplastic.