Programmed Death-1 (PD-1) is a type I transmembrane protein that is expressed on T cells, B cells and monocytes during activation. PD-1 and its ligands (PD L1 and PD-L2) function as an immune checkpoint signaling pathway by regulating T cell activation and autoimmunity. PD-1 labels follicular helper T cells and is therefore a useful marker for angioimmunoblastic T-cell lymphoma. It is mainly used in follicular lymphoma and other low-grade malignant medium and small B-cell lymphomas, clear cell renal carcinoma and other clear cell carcinomas, endometrial stromal sarcoma and uterine leiomyosarcoma.