In the present study, to address this critical issue, we have improved the screening methods and succeeded in developing novel anti-human CD26 mAbs with strong binding affinity to denatured human CD26 in FFPE non-tumor and tumor tissue sections, and which do not stain CD26-negative specimens, suggesting that these novel mAbs are potentially useful for the analysis of CD26 expression in cancer patients, and may help decide the appropriateness of YS110 therapy for future cancer patients. This evidence concerns the gene DPP4 and cancer.