KLK3 and head and neck squamous cell carcinoma: Similar to α-fetoprotein in hepatic cell carcinoma [65], prostate-specific antigen in prostate cancer [66], or human chorionic gonadotropin-β in gestational trophoblastic tumors [67], squamous cell carcinoma antigen (SCC-Ag) is a potential biomarker for HNSCC, whereas an elevated level of SCC-Ag also suggests other diseases besides HNSCC, such as skin inflammation, autoimmune diseases and malignancies [68,69,70].