EGFR and head and neck squamous cell carcinoma: EGFR is expressed in the skin, gastrointestinal system, kidney, and other normal tissues at the physiological level; however, it is aberrantly activated in many epithelial tumors, such as lung cancer, pancreatic cancer, colorectal cancer, breast cancer, and head and neck squamous cell carcinoma (HNSCC) [32, 33].