PD-1 is a cell surface membrane protein of the immunoglobulin superfamily. It is an inhibitory receptor on antigen activated T-cells that plays a critical role in induction and maintenance of immune tolerance to self. PD-1 delivers inhibitory signals upon binding to ligands CD274/PD-L1 and CD273/PD-L2. [taken from the Universal Protein Resource (UniProt) www.uniprot.org/uniprot/Q15116].