Immune checkpoint inhibitors are revolutionizing cancer therapeutics, including all those targeting the programmed cell death 1/programmed cell death ligand 1 and cytotoxic T lymphocyte antigen 4 pathways. Both activity and toxicity stem primarily from the release of tumor-specific or host-specific cytotoxic T cells. In immune checkpoint inhibitor clinical trials, several patients seen in routine clinical practice have not eligible for or been seriously underrepresented. Thus, there is a significant gap in knowledge on the effectiveness and safety of these agents, even after regulatory approval. (Johnson DB et al. 2017)