A study suggested that hypoxia could drive intravasation via signaling through CXCR4 and its ligand stromal cell derived factor 1 (SDF-1, also known as C-X-C motif chemokine ligand 12 (CXCL12)), which results in adhesion of cancer cells to endothelial cells and trans-endothelial migration of breast cancer cells in in vitro assays [157]. Here, CXCL12 is linked to cancer.