This study reveals that DPP9 inhibits ubiquitin-mediated degradation of NRF2 protein by binding to KEAP1, up-regulates NRF2 protein levels, promotes mRNA transcription of NQO1, inhibits intracellular ROS levels, and thus weakens the responses of liver cancer to chemotherapy drugs (Fig. 8). This evidence concerns the gene NQO1 and liver cancer.