STAT3 and head and neck squamous cell carcinoma: Although its phosphorylation is transient and tightly regulated in normal non-transformed cells, STAT3 is persistently activated in a variety of human tumors, including hematological malignancies (leukemia, multiple myeloma and lymphoma) and solid tumors (head and neck squamous cell carcinoma [HNSCC], melanoma, colon, hepatoma, breast, and prostate cancers) [13-17].