Protein CIP2Aalso known ascancerous inhibitor of PP2A(CIP2A) is aproteinthat in humans is encoded by theKIAA1524gene. Protein phosphatase 2A (PP2A) is a cellular tumor suppressor which inhibits cell proliferation and transformation of normal cells to malignant cancer cells. Inhibition of PP2A activity is a prerequisite for human cell transformation. However, the in vivo mechanisms by which PP2A activity is inhibited in spontaneously transformed human cancer cells have been unclear.