The transmembrane protein PD-1 is known to be expressed on the surface of a variety of immune cells, including activated T cells like CD4+ and CD8+ T cells (cytotoxic T lymphocytes, or CTLs), while PD-L1 is expressed in tumor cells, and its interaction with PD-1 prevents the immune system-mediated killing of cancer cells21. Here, PDCD1 is linked to neoplasm.