PDCD1 and cancer: Discoveries of programmed cell death 1 (PD-1) [23] and cytotoxic T-lymphocyte antigen 4 (CTLA-4) [24] were major breakthroughs, and the disclosure of their roles [25–28] in tumor immune environments has led to the creation of ICI [6, 29, 30], which caused a paradigm shift in cancer treatments.