PDCD1 and hepatocellular carcinoma: In hepatocellular carcinoma patients, circulating NKT-like cells are functionally exhausted and are characterized by downregulation of NKG2D, upregulation of programmed cell death protein 1 (PD-1), T-cell immunoglobulin and mucin structural domain protein 3 (Tim-3), and cytotoxic T-lymphocyte antigen 4 (CTLA-4), and impaired functionality 9.