PDCD1 and neoplasm: One study by Zeng et al. tested the efficacy of a chemokine receptor 4 (CXCR4) antagonist (AMD3100) in combination with an anti-PD-1 monoclonal antibody in vivo [77] CXCR4’s interaction with its ligand chemokine stromal cell-derived factor-1 (CXCL12) plays a crucial role in promoting tumor invasion and metastasis via promotion of myeloid bone marrow-derived cells, CAFs, and angiogenesis [78].