PDCD1 and cancer: With the hope of boosting the patient’s immune system to kill cancer, one strategy has been to use immunotherapy targeting PD-1/PD-L1, cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), lymphocyte-activation gene 3 (LAG-3), T cell immunoglobulin and mucin-domain containing-3 (TIM-3) and the hedgehog (Hh) and neuropilin-2 (NRP-2) signaling pathway (reviewed in [2]).