PDCD1 and cancer: High inhibitory ligand expression is a hallmark of many cancers.113, 114, 115 In infectious disease, they are most frequently studied as markers of T cell exhaustion after viral infection.116, 117, 118, 119, 120 Three of the most commonly studied inhibitory ligands are programmed cell death 1 (PD‐1), cytotoxic T‐lymphocyte associated protein (CTLA‐4), T cell immunoglobulin and mucin‐domain containing 3 (TIM‐3).