In addition, TK1 has been evaluated as a marker of tumor proliferation, primarily through immunohistochemistry and functional imaging using 18F-fluorothymidine (18F-FLT) positron emission tomography (PET) in head and neck squamous cell carcinoma (HNSCC) [11,12,13]. The gene discussed is TK1; the disease is head and neck squamous cell carcinoma.