PDCD1 and cancer: Important biomarkers predictive of clinical response to immunotherapy in melanoma and other cancers include high tumor mutation burden [2,3], tumor infiltration with CD8 T cells [4,5], tumor expression of the PD1 ligand, PD-L1 [6], immune-related signatures of local T-cell activation and interferon (IFN)γ response [5,7], and early on-treatment changes associated with T-cell activation and IFNγ release [8,9].