The metastatic organotropism can be explained by the type of complementary chemokines and their receptors expressed in the target endothelium of vasculature and CTCs.164 Among all chemokines, CXC-chemokine ligand 12 (CXCL12) has been well investigated in extravasation and metastasis of cancer cells.165 CXCL12 released by stromal cells at the distant tissues can draw tumour cells expressing CXCR4 and CXCR7 to activate extravasation, migration, and adhesion of cancer cell. This evidence concerns the gene CXCR4 and cancer.