PDCD1 and hepatocellular carcinoma: Single-cell RNA sequencing (scRNA-seq) studies of immune cells in HCC tumors have recently revealed a subpopulation of CD8+ T cells which expressed high levels of markers, such as cytotoxic T lymphocyte-associated protein 4 (CTLA4), PDCD1 (also called PD1), and HAVCR2 (also called TIM3) [91].