CD117: Definition

CD117 (also known as c-kit or KIT) is a protein found on the surface of certain normal and abnormal cells. It acts like an antenna — receiving signals from the surrounding environment that tell the cell to grow, divide, and survive. In pathology, CD117 is used as an immunohistochemical (IHC) marker, meaning it is detected in tissue samples using a special laboratory stain. Pathologists use CD117 to help identify specific tumor types, most importantly gastrointestinal stromal tumors (GISTs). In some tumors, a permanent change in the gene that makes CD117 — called a mutation — keeps the protein on like a light switch stuck in the on position. This drives uncontrolled cell growth and is directly relevant to treatment decisions.

Why do pathologists test for CD117?

CD117 is tested for two main purposes:

To confirm the diagnosis of specific tumors. CD117 is strongly present in certain tumor types — most importantly GISTs and tumors arising from mast cells (a type of immune cell involved in allergic reactions). Identifying CD117 in a tumor helps the pathologist confirm the type of cancer.
To identify candidates for targeted therapy. In GISTs, most tumors have a permanent gene change (mutation) in the KIT gene — the gene that makes CD117 — that keeps the protein constantly active. A positive CD117 result, combined with gene testing, helps identify patients who may benefit from drugs specifically designed to block this abnormal signal, such as imatinib (Gleevec).

Which tumors are commonly CD117 positive?

CD117 is most important in the following settings:

Gastrointestinal stromal tumor (GIST) — the most important setting for CD117 testing. The great majority of GISTs are strongly CD117 positive. Most carry a mutation — a permanent change in the DNA — in the KIT gene that keeps CD117 switched on, driving tumor growth. This finding is central to both confirming the diagnosis and planning treatment.
Mast cell tumors and mastocytosis — mast cells normally carry CD117 on their surface. Tumors arising from mast cells, including a condition called systemic mastocytosis — where too many mast cells accumulate in the bone marrow and other organs — are consistently CD117 positive. Many cases carry a specific gene mutation, D816V, that keeps CD117 permanently active and is associated with resistance to standard imatinib doses.
Acute myeloid leukemia (AML) — a cancer of the blood and bone marrow. CD117 is found on immature blood cell precursors called blasts — abnormal cells that have not developed properly and build up in the bone marrow. Its presence helps identify these cells and classify the leukemia.
Germ cell tumors — tumors that arise from reproductive cells. Seminomas (which develop in the testis) and dysgerminomas (which develop in the ovary) are typically CD117-positive, which can help confirm where the tumor originated.
Melanoma and other tumors — CD117 may be present in melanoma (a type of skin cancer arising from pigment-producing cells) and several other tumor types, though it is less specific in these settings and is always interpreted alongside other test results.

How is CD117 tested?

Pathologists detect CD117 using immunohistochemistry. A thin slice of tumor tissue is placed on a glass slide and treated with an antibody — a protein designed to attach specifically to CD117. Where CD117 is present, the antibody triggers a color change (typically brown) visible under the microscope. The color appears most often in the cytoplasm — the main body of the cell, surrounding the nucleus — and sometimes also on the outer surface of the cell.

How results are reported

CD117 results are reported as positive or negative, often with a description of how strongly the cells stain and what proportion of cells are affected:

Positive (reactive) — CD117 protein is detected in the tumor cells. In a tumor from the digestive tract being evaluated for GIST, a strongly positive CD117 result is strong evidence supporting this diagnosis and usually leads to further gene testing to identify the specific mutation and guide treatment.
Negative (non-reactive) — CD117 protein is not detected. A negative result in a suspected GIST does not completely rule out the diagnosis. A small number of GISTs do not express CD117, particularly those driven by a mutation in a related gene, PDGFRA, which plays a similar role to KIT in regulating cell growth. In these cases, another marker called DOG1 — a protein also characteristic of GISTs — may be tested to help confirm the diagnosis.

What does a CD117 result mean for treatment?

In GIST, a positive CD117 result is closely linked to treatment decisions. Most CD117-positive GISTs carry a KIT gene mutation that makes the tumor sensitive to imatinib (Gleevec) — a targeted therapy that blocks the abnormal signal driving tumor growth. The specific location of the mutation within the KIT gene — found in a particular section called an exon — helps predict how well imatinib will work and whether a standard or higher dose is needed. GISTs driven by PDGFRA gene mutations rather than KIT mutations may behave differently and require different treatments.

If your report mentions CD117 in the context of GIST, your oncologist will typically arrange gene testing to identify the specific mutation and select the most appropriate treatment.

Questions to ask your doctor

Was CD117 positive in my tumor, and what does this mean for my diagnosis?
Has gene testing been performed to identify a KIT or PDGFRA mutation?
Am I a candidate for imatinib or another targeted therapy?