FAP and cancer: FAP-positive CAFs in tumor stroma are the major source of C-X-C motif ligand 12 (CXCL12) in the tumor, and inhibition of its receptor, the C-X-C motif receptor 4 (CXCR4), induced T cell accumulation in the tumor and acted synergistically with anti-PD-L1 in diminishing cancer cells [107].