p16: Definiton

p16 is a protein produced by cells throughout the body that helps control how cells grow and divide. Because it acts as a brake on cell growth, p16 is known as a tumor suppressor protein — a protein whose normal job is to prevent cells from growing uncontrollably. In pathology, p16 is most commonly used as a marker for cancers and precancerous conditions caused by human papillomavirus (HPV) infection. When a pathologist tests for p16, they are usually looking for strong, widespread staining that points to an HPV-related cause.

Why is p16 linked to HPV infection?

At first, it can seem confusing that a cancer would show more of a tumor suppressor protein. The explanation lies in how HPV affects the cell.

Normally, p16 works alongside another protein called the retinoblastoma protein (Rb), which helps keep cell growth in check. When the Rb protein is working properly, the cell produces only small amounts of p16. High-risk types of HPV produce a viral protein that switches off the Rb protein. When Rb is switched off, the cell responds by producing large amounts of p16 to regain control of cell growth — but because Rb is disabled, this effort fails, and the cell continues to divide abnormally.

The end result is that cells infected by high-risk HPV become packed with p16 protein. This is why strong, widespread p16 staining is such a useful clue: it is an indirect sign that high-risk HPV is driving the abnormal cell growth. For this reason, p16 is described as a surrogate marker for HPV infection — it does not detect the virus itself, but it reliably indicates its effects on the cell.

How is p16 tested?

Pathologists test for p16 using a laboratory method called immunohistochemistry (IHC). An antibody — a protein designed to attach specifically to p16 — is applied to a thin slice of tissue on a glass slide. Where p16 is present, the antibody produces a visible color change that can be seen under the microscope. The staining appears in both the nucleus (the part of the cell that contains DNA) and the cytoplasm (the body of the cell surrounding the nucleus) of affected cells.

How results are reported

The way p16 results are reported is important, because not all positive staining means the same thing:

Block-positive (strong and diffuse) — strong staining seen across most of the tumor cells, involving both the nucleus and cytoplasm. This pattern is the one pathologists look for as evidence of an HPV-related cause. It supports a diagnosis of an HPV-driven precancer or cancer.
Patchy or focal positive — staining seen only in scattered cells or small areas. This pattern is generally not considered evidence of HPV-related disease and is interpreted as negative for the purpose of HPV testing.
Negative — little or no p16 staining. In a precancerous lesion of the cervix or other anogenital site, a negative result suggests the changes are not high-grade or HPV-driven.

Because the pattern of staining matters so much, your pathology report will usually describe not just whether p16 is positive, but how strong and widespread the staining is.

Which conditions are commonly p16 positive?

Strong, diffuse p16 staining is most often seen in precancerous and cancerous conditions caused by high-risk HPV, including:

High-grade squamous intraepithelial lesion (HSIL) — a precancerous change in the squamous cells of the cervix, vagina, vulva, anus, or penis. Strong p16 staining helps confirm that a lesion is high-grade rather than a lower-grade change. (See also squamous intraepithelial lesion.)
Squamous cell carcinoma of the cervix, anus, vulva, vagina, and penis — HPV-related squamous cancers of the anogenital region are typically strongly p16 positive.
Squamous cell carcinoma of the oropharynx — cancers of the throat, tonsils, and base of the tongue. p16 testing is a routine and important part of the workup for these cancers (see below).
Cervical adenocarcinoma and adenocarcinoma in situ — HPV-related glandular cancers and precancers of the cervix are also typically p16 positive.

Strong p16 staining can occasionally be seen in some cancers not caused by HPV, including certain cancers of the female reproductive tract, such as uterine serous carcinoma and high-grade serous carcinoma of the ovary. In these cancers, p16 is not acting as an HPV marker; instead, it reflects a different underlying genetic change. This is why p16 results are always interpreted in the context of the specific tumor type and its location.

Why does p16 matter for oropharyngeal cancer?

In squamous cell carcinoma of the oropharynx, p16 testing has become especially important because it does more than confirm the diagnosis — it provides valuable information about prognosis. HPV-related (p16-positive) oropharyngeal cancers generally respond better to treatment and have a significantly more favorable outlook than oropharyngeal cancers that are not related to HPV. For this reason, p16 status is used to help classify these cancers and is taken into account when planning treatment. If your oropharyngeal cancer report includes a p16 result, it is worth discussing with your doctor what that result means for your specific treatment plan and prognosis.

Questions to ask your doctor

What does the p16 result in my report mean, and does it indicate that my condition is related to HPV?
Was the p16 staining strong and diffuse (block-positive), and how does that affect my diagnosis?
Does my p16 result affect my prognosis or my treatment plan?