PDCD1 and cancer: Programmed cell death protein 1 (PD‐1/PDCD1), programmed cell death 1 ligand 1 (PD‐L1/CD274), hepatitis A virus cellular receptor 2 (HAVCR2/TIM3), programmed cell death 1 ligand 2 (PDCD1LG2), and T‐cell immune‐receptor with Ig and ITIM domains (TIGIT) are vital immune checkpoint proteins that are promising cancer immunotherapy targets.