October 25, 2023

This picture shows S100 protein (brown) in a normal nerve.
This picture shows S100 protein (brown) in a normal nerve.

S100 is a protein that is made by many different types of normal, healthy cells including cells in the skin, salivary glands, fat, cartilage, and nerves. Tumours that start from these types of tissue may also make S100. Once the S100 protein is made, it is held in both the nucleus and cytoplasm of the cell. The nucleus is the part of the cell that holds most of the cell’s genetic material (DNA) while the cytoplasm is the body of the cell that surrounds the nucleus.

Pathologists can see S100-producing cells by performing a test called immunohistochemistry (IHC). The test result is often reported as positive (the cells are making the protein) or negative (the cells are not making the protein). In this skin, this test is often used to identify normal melanocytes or tumours made up of melanocytes such as melanoma. In other parts of the body, this test aids in the diagnosis of various types of tumours.

Examples of benign (noncancerous) tumours that are positive for S100:
Examples of malignant (cancerous) tumours that are positive for S100:

About this article

This article was written by doctors to help you read and understand your pathology report. Contact us if you have questions about this article or your pathology report. For a complete introduction to your pathology report, read this article.

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