PDCD1 and cancer: The effectiveness of immunotherapy depends on the ability of checkpoint modulators to unleash pre-existing immunity, specifically T-cell effector functionality.20,21 T cells from the lymphoid compartment have been shown to regulate metastasis.22–24 Inhibiting the checkpoint modulators is an important tool in cancer immunotherapy and well-established checkpoint molecules include programmed cell death 1 (PD-1), programmed cell death 1 ligand 1 (PD-L1), cytotoxic T lymphocyte-associated protein 4 (CTLA-4), T-cell immunoglobulin and mucin domain 3 (TIM-3) and lymphocyte activation gene 3 (LAG3).