p16 is a protein made by normal cells throughout the body. It can also be made by abnormal cells including some types of cancers. It is a type of protein called a tumour suppressor. This means that the normal role of p16 is to stop the cell from changing into a cancer cell.
After it is made, most of the p16 stays in a part of cell called the nucleus. The nucleus is the part of the cell that holds most of our genetic information. Pathologists use a test called immunohistochemistry to see p16 inside cells. The results of this test are used with other information to make a diagnosis.
Some types of cancers can make extra p16. When immunohistochemistry is performed, pathologists call this result over expression. Pathologists use this result to help them decide the cancer type.
Cells that have been infected by a virus called human papillomavirus (HPV) can also make extra p16. In certain areas of the body, pathologists look for extra p16 inside groups of cells when trying to decide if the changes they are seeing under the microscope are caused by HPV.