PLK1 and head and neck squamous cell carcinoma: PLK1 is highly expressed in proliferating cancer cells, including breast cancer [5], colorectal cancer [6], esophagus and stomach cancer [7], endometrial carcinomas [8], head and neck squamous cell carcinomas [9], non-small cell lung cancer [10], ovarian cancer [11], pancreatic cancer [12] and skin cancer [13] etc. In some types of tumors, overexpression of PLK1 correlates with a poor prognosis.