DPP4 and lung cancer: In addition to its function as an immunoregulatory molecule, CD26 is expressed on a number of human neoplasms, including malignant pleural mesothelioma (MPM), renal carcinoma (RCC), colorectal cancer (CRC), hepatocellular carcinoma, lung cancer, prostate cancer, gastrointestinal stromal tumor (GIST), thyroid carcinoma, and different subtypes of T-cell malignancies [9].