Tissue Polypeptide Antigen (TPA) is a well-known tumor activity marker that has been on the market for over twenty years. IDL offers TPAcyk™ a quantitative serum monoclonal immunoassay reactive with soluble cytokeratins 8 and 18 in serum. TPAcyk™ is used in the management of patients with different epithelial cell carcinomas. The tumor activity marker adds valuable information to the overall clinical picture for establishing prognosis, in treatment monitoring, and in follow-up of patients after completed therapy.