PDCD1 and cancer: Anti-tumor T-cell responses can control clinical progression of cancer, as exemplified by the effects of T-cell targeting immunotherapies such as immune checkpoint inhibition, including antibodies against cytotoxic T-lymphocyte-associated protein 4 (CTLA4) or programmed cell death 1 (PD-1), high-dose IL-2 treatment, or adaptive T-cell immunotherapy (1–3).