PDCD1 and hepatocellular carcinoma: Nivolumab, a human anti-PD-1 (programmed cell death protein 1) IgG4 monoclonal antibody that inhibits PD-1, received US Food and Drug Administration (FDA) approval in 2017 as a second-line therapy for HCC patients who had experienced disease progression after initial sorafenib treatment (Wong et al. 2021).