p40 is a protein that is made by many different kinds of normal healthy cells including squamous cells, basal cells, and myoepithelial cells. Tumours that start from these types of specialized cells also make p40.
Once the p40 protein is made it is held in a part of the cell called the nucleus. The nucleus is an important part of the cell because it holds most of the cells’ genetic material (DNA).
Pathologists use a test called immunohistochemistry to ‘see’ the p40 inside these cells. Pathologists will perform this test when they are trying to identify the cells in a tissue sample as one of these types of cells. Cells that make p40 will be called “positive” or “reactive”.