To investigate whether DPP9 affects the responses of liver cancer cells to chemotherapy drugs, MTT assay was used to detect the toxic effects of chemotherapy drugs (cisplatin, doxorubicin, and 5-FU) on SK-Hep-1 and HepG2 cells with DPP9 overexpression or silencing (Fig. 1A and B). Here, DPP9 is linked to liver cancer.