PDCD1 and cancer: Gene aberrations in antigen processing machinery (APM), including tumor-intrinsic loss of major histocompatibility complex class I (MHC-I), and mutations in the IFNγ signaling pathway are well-documented hallmarks of resistance to PD-1/PD-L1 blockade [14–17] in various cancers and can potentially hinder patient response to other immunotherapies, including adoptive T cell therapies and therapeutic cancer vaccines.