p16

p16 is a widely used immunohistochemical marker indicating expression of the cell cycle protein cyclin-dependent kinase inhibitor 2A, which is upregulated by human papillomavirus (HPV) infection. In the uterine cervix, p16 positivity supports the diagnosis of a high-grade squamous intraepithelial lesion (HSIL). Testing p16 positive in a squamous cell carcinoma of the head and neck indicates an HPV-related etiology and confers, particularly in the oropharynx, a better prognosis.