PDCD1 and cancer: Notably, aberrant expression of immune checkpoints, including PD-1 and its ligands (PD-L1/2), CTLA-4, V-domain immunoglobulin suppressor of T-cell activation (VISTA), T-cell immunoglobulin and mucin-domain containing-3 (TIM-3) and lymphocyte-activation gene 3 (LAG-3) has all been detected in a variety of human cancers [11].