To determine the underlying molecular mechanism of USP21 function in promoting both normal hepatocyte and HCC growth, we immunoprecipitated the FLAG-tagged USP21 from lysates of HEK-293T cells transfected with USP21 and identified the co-immunoprecipitated proteins with USP21-FLAG by mass spectrometry (Fig. 4a). This evidence concerns the gene USP21 and hepatocellular carcinoma.