PDCD1 and neoplasm: PD-1 is a T cell coinhibitory receptor, which is expressed on a large proportion of tumor-infiltrating lymphocytes in many different tumor types;9 its major ligand PD-L1 (also known as B7-H1 and CD274)10,11 is selectively expressed on many tumors,12,13 including PDAC, and on cells within the TME, such as macrophages and the stromal cell subset, in response to inflammatory stimuli.14