Inhibition of protein phosphatase 2A (PP2A) activity has been identified as a prerequisite for the transformation of human cells. The protein, designated Cancerous Inhibitor of PP2A (CIP2A, p90 Autoantigen), interacts directly with the oncogenic transcription factor c-Myc, inhibits PP2A activity toward c-Myc serine 62 (S62), and thereby prevents c-Myc proteolytic degradation. In addition to its function in c-Myc stabilization, p90 Autoantigen promotes anchorage-independent cell growth and in