Malignant cells in the tumors generate TGF-β, thus upregulating CXCR4 on the monocyte cell surface, whereas perivascular fibroblasts produce the CXCR4 ligand CXCL12 to target these monocytes to the tumor, followed by monocytes differentiating into perivascular macrophages (142). The gene discussed is CXCR4; the disease is neoplasm.