PDCD1 and hepatocellular carcinoma: For example, epigenetic regulation including DNA methylation, histone methylation, histone acetylation, and histone ubiquitination were proved to target programmed cell death protein 1 (PD1)/programmed cell death lidand 1 (PDL1), and these epigenetic regulations were found to be critical in the immunotherapy of malignant melanoma and hepatocellular carcinoma (HCC).4, 5