PDCD1 and cancer: In 1999, the research group of Lieping Chen at the Mayo Clinic discovered a molecule named B7-H1,38 which was later found to be expressed on tumor tissues such as melanoma and lung cancer and can promote the apoptosis of tumor-specific T cells, making them unable to attack cancer cells.39 In 2000, B7-H1 was identified as a ligand of PD-1, therefore acquiring its second name PD-L1.40 In 2002, PD-L2 was discovered, and the signaling pathway involving PD-1 was clarified.41,42 These discoveries demonstrated that PD-1 is another IC.