In the last decade, breakthroughs in cancer immunotherapy have brought substantial survival benefits to some patients by upregulating anti-tumor immunity through immune checkpoint blockade targeting cytotoxic T lymphocyte-associated protein 4 (CTLA4), programmed cell death protein 1 (PD-1), or the programmed cell death 1 ligand 1 (PD-L1) (Couzin-Frankel, 2013; Sun et al., 2018; Kamimura et al., 2019). Here, PDCD1 is linked to cancer.