CXCL9 is thought to be a key chemokine in anti-PD-1 antibody treatment, since the number of tumor-infiltrating CD8+ T cells induced by anti-PD-1 antibody treatment was lower in both CXCL9 knockout and CXCR3, the receptor of CXCL9, knockout mice bearing MC38 tumors, resulting in an impaired effect on tumor growth22. This evidence concerns the gene PDCD1 and neoplasm.