The clone 6D5, a mouse monoclonal antibody selectively binds with a 50-55 kD cell surface protein commonly known as Programmed cell death 1 (PD-1) or CD279, a member of the immunoglobulin superfamily. PD-1 expression is mostly observed in activated T cells and B cells, and also in dendritic cells. PD-1 signals via binding its two ligands, PD-L1 and PD-L2. Upon ligand binding, PD-1 signaling inhibits T-cell activation, leading to reduced proliferation, cytokine production, and T cell death. Blocking of PD-1 by its antibody restores T cells immunity against tumor and infectious agents. PD-1-blockade based immunotherapy is, therefore, highly clinically useful against various types of cancers and infectious diseases.