CXCR4 and head and neck squamous cell carcinoma: Associations between SDF-1/CXCR4 polymorphisms or SDF-1/CXCR4 abundance in tumor specimens and clinical outcome in several but not all studies might suggest a role of SDF-1/CXCR4 signaling in metastatic progression in a variety of tumor entities, such as renal cell carcinoma (97), prostate cancer (98), HNSCC (99–102), esophagogastric cancer (103), colorectal cancer (74), hepatocellular carcinoma (104), or osteosarcoma (105).