Programmed death 1 (PD-1) is a cell surface co-receptor in the CD28/CTLA-4 T cell family and functions as a down regulator of the immune system through a dual mechanism of inhibition (1). PD-1 is expressed on the cell surface of activated T- and B-cells. Anti-tumor immunity may be controlled by the PD-1/PD-L1 signaling pathway. PD-L1, one of the ligands associated with PD-1, provides immunity for tumor cells by inducing apoptosis of activated T cells or by inhibiting cytotoxic T cells (1,2). Therapies that target the PD-1 receptor have shown unprecedented results with high levels of clinical response in patients with various cancer types (3). The presence of PD-1 positive tumor-infiltrating lymphocytes (TIL) has been associated with poor prognosis in human breast cancers and may be useful in antibody therapy targeting the PD-1/PD-L1 signaling pathway (1). Treatments targeting PD-1 and its ligand, PD-L1, have also shown encouraging results in non-small-cell lung cancer, renal cell carcinoma and melanoma (4-6).