Further, as a possible mechanism of tumor-to-tumor metastasis, the involvement of both C-X-C motif chemokine ligand 12/stromal cell-derived factor-1 (CXCL12/SDF-1) and C-X-C motif chemokine receptor 4 (CXCR4) is examined. This evidence concerns the gene CXCL12 and neoplasm.