PDCD1 and cancer: Its effects on immune effectors are many: hypoxia has been recognized as an upregulator of the expression of PD-1/PD-L1 on tumor cells via HIF-1α (Noman et al., 2014), and impairing hypoxia sensitizes cancer cells to T cell checkpoint blockade with CTLA-4 and PD-1 (Jayaprakash et al., 2018).