CXCR4 and cancer: There appear to be two mechanisms by which CXCL12 affects tumor cell biology: 1) direct stimulation of signaling pathways that promote cancer cell growth, metastasis, and angiogenesis; 2) indirect effects, including the recruitment of CXCR4/CXCR7-positive cancer cells to CXCL12-expressing organs (Duda et al., 2011).