As shown in Fig. 6A and B, there was no discernible difference in the rate of NQO1 protein degradation in SK-Hep-1 and HepG2 cells following the overexpression and silencing of DPP9, indicating that DPP9 does not affect the levels of NQO1 protein in liver cancer cells by regulating the stability of NQO1 protein. This evidence concerns the gene NQO1 and liver cancer.