PDCD1 and neoplasm: Belonging to the immunoglobulin super family, PD-1 is a transmembrane coinhibitory receptor primarily expressed on the surface of activated T cells and NK cells (Huang et al., 2021) as the ligands to PD-1, PD-L1 (B7-H1 or CD274), and PD-L2 (B7-DC or CD273) are expressed on the surface of tumor cells (Figure 2) (Huang et al., 2021).