Human CD26-positive or negative tumor cell lines were incubated with a sufficient amount (10 μg/ml) of unlabeled mouse anti-human CD26 mAb (19–32, U16-3 or U38-8) or control pAb or isotype controls, and subsequently stained with PE-labeled anti-mouse Ig pAb or PE-labeled anti-goat IgG Ab. Here, DPP4 is linked to neoplasm.