Programmed Death 1 (PD-1) is a member of the CD28/CTLA-4 family of T-cell regulators, expressed as a co-receptor on the surface of activated T-cells, B-cells, and macrophages. New studies have suggested that the PD-1/PD-L1 signaling pathway may be linked to anti-tumor immunity, as PD-L1 has been shown to induce apoptosis of activated T cells or inhibit activity of cytotoxic T cells. In comparison to CD10 and Bcl-6, PD-1 is expressed by fewer B cells and has therefore been considered a more specific and useful diagnostic marker for angioimmunoblastic T-cell lymphoma. Therapies targeted toward the PD-1 receptor have shown remarkable clinical responses in patients with various types of cancer, including non–small-cell lung cancer, melanoma, and renal-cell cancer.