Many studies have reported that DPP9 is overexpressed in human cancer and cancer cell lines, including ovarian carcinoma, testicular tumours, hepatocellular carcinoma, clear cell renal carcinoma, colorectal cancer, and non-small cell lung cancer, suggesting that inhibiting DPP9 may reduce tumour growth [27, 44, 75, 104–108]. Here, DPP9 is linked to hepatocellular carcinoma.