For example, while activated CD8+/PD-1+ CTLs can infiltrate tumors, they are frequently thwarted in their anti-cancer role by tumor cell expression of PD-1 ligand 1 (PD-L1), directly stimulated by key CTL cytokines such as IFNγ, triggering a PD-1 dependent deactivation state known as ‘adaptive resistance’ [33]. Here, PDCD1 is linked to cancer.