NEDD8 and hepatocellular carcinoma: Co-IP and western blot experiments showed that in HCC cells transfected with HA-PCNA plasmids, NEDD8 overexpression increased PCNA NEDDylation and decreased PCNA K48-linked polyubiquitination accompanied by an increase in PCNA protein expression compared with the control group and NEDD8 knockdown yielded the opposite results (Fig. 5E and Supplementary Fig. 3D).