Evidence of the role of reducing immune system function and promoting self-tolerance by canceling the inflammatory activity of T lymphocytes by programmed cell death protein 1 (PD-1), expressed on the surface of activated T lymphocytes, clarified the role of this protein in the prevention of autoimmune diseases, but also in promoting the expansion of cancer cells [18,23,24,25,26,27]. The gene discussed is PDCD1; the disease is cancer.