PDCD1 and neoplasm: Programmed cell death ligand 1 (PD-L1) is a molecule expressed on tumour cells or on tumour-infiltrating immune cells, which binds programmed cell death protein 1 (PD-1) expressed on effector CD8+ T lymphocytes, blocking the secretion or production of the cytotoxic mediators needed to kill tumour cells within the tumour beds.