CXCL12 and breast cancer: At present, 23 different cancer types have been shown to overexpress CXCR4 and to respond to its ligand CXCL12, including breast cancer, ovarian cancer, small-cell lung cancer, hepatocellular carcinoma, gastric and stomach cancers, melanoma, brain tumors, soft tissue sarcomas, pancreatic cancer, brain cancer, and ovarian cancer [25,26].