Then, we tested whether preincubation with CXCL12 before adding [18F]MSX-122F blocked binding to CXCR4 in the CXCR4-positive metastatic squamous cell carcinoma of head and neck (SCCHN) cells that we established from a poorly metastatic parental cell line by four rounds of in vivo selection using a lymph node metastatic xenograft mouse model. The gene discussed is CXCL12; the disease is head and neck squamous cell carcinoma.