CIP2A (cancerous inhibitor of PP2A) was originally identified as p90, a cytoplasmic autoantigen found in the serum from a cancer patient. It was later described as a protein that interacted with the c-myc oncogene and functioned as an inhibitor of protein phosphatase 2A (PP2A) activity. Inhibition of PP2A phosphatase activity by CIP2A prevents c-myc phosphorylation and counteracts c-myc proteolytic degradation, thereby stabilizing c-myc and promoting cell growth and transformation. CIP2A is found to be overexpressed in some human malignancies and is thus classified as an oncogene that may be an important contributor to tumor formation.