The association between the gut microbiota composition and clinical outcomes of immune checkpoint inhibitors (ICIs), such as inhibitors targeting cytotoxic T lymphocyte antigen-4 (CTLA-4), programmed cell death protein 1 (PD-1), and its ligand (PD-L1), was demonstrated in various cancer cohorts [1,2,3,4,5]. This evidence concerns the gene PDCD1 and cancer.