Immune checkpoint inhibitors including ipilimumab and tremelimumab, anti-cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) antibodies, and nivolumab and pembrolizumab, anti-programmed cell death protein 1 (PD-1) antibodies, are recently developed cancer immunotherapy agents which activate T lymphocytes and enhance immune responses to cancers [1–4]. Here, PDCD1 is linked to cancer.