PDCD1 and neoplasm: However, these CD8+ T cells may become dysfunctional or ‘exhausted’, characterized by impaired effector functions, increased expression of inhibitory receptors, such as programed cell death protein 1 (PD1), programmed death ligand 1 (PD-L1), T-cell immunoglobulin and mucin domain-containing protein 3 (TIM-3), contributing to tumor immune evasion14.