In transformed cells, inactivation of PP2A phosphatase and, therefore, loss of its tumour suppressor activity may not only be a consequence of genetic alterations (i.e. mutations) and/or aberrant expression of a specific PP2A subunit, but it could also derive from aberrant activation of signal-transduction pathways leading to post-translational modifications of the PP2A catalytic subunits and/or increased expression of molecules that act as general physiological inhibitors of PP2A phosphatase activity (Janssens and Goris, 2001). Here, PTPA is linked to neoplasm.