PDCD1 and hepatocellular carcinoma: As one of the components of bile acid, Nor Cholic acid (NorCA) can promote the immune escape of liver cancer cells and obviously increase Programmed Cell Death-1 (PD-1) on the surface of CD4+ T cells by destroying the FXR/SHP signal axis, accompanied by upregulation of Programmed Cell Death Ligand-1 (PD-L1) in hepatocellular carcinoma (HCC) cells and their secreted exosomes.