We further examined immunohistochemical staining of CD26-expressing tumor tissues other than malignant mesothelioma (hepatocellular carcinoma, renal cell carcinoma, prostate adenocarcinoma, colon adenocarcinoma, and lung adenocarcinoma) with the purified mAb of clone 18 or 19. The gene discussed is DPP4; the disease is malignant mesothelioma.