PDCD1 and hepatocellular carcinoma: Despite the great improvements on treatment approaches for HCC in recent decades, such as from laparotomy liver resection to laparoscopic hepatectomy or even liver transplantation, radiofrequency ablation to transcatheter arterial chemoembolization, and sorafenib to programmed cell death-1, they had little effect to hinder the increasing mortality year by year [25].