Biomarker testing

• What they are: Proteins on the surface of cells that pick up growth signals from the hormones estrogen and/or progesterone.
• Their function in breast cancer: When estrogen and/or progesterone are present, hormone receptors help breast cancer cells grow and multiply.
• Biomarker test used: An immunohistochemistry (IHC) test, a type of staining technique used on tumor tissue; testing is routinely done after a biopsy. Testing can also be done if cancer recurs or metastasizes to confirm whether receptor status has changed.
• How a positive result guides treatment: Hormone receptor-positive breast cancers can be treated with endocrine therapy, which interferes with estrogen’s ability to help the cancer grow. Examples include tamoxifen and aromatase inhibitors. In addition, hormone receptor-positive breast cancer may be eligible for treatment with certain types of targeted therapy.

• What they are: Proteins on the surfaces of cells that pick up growth signals from the HER2 gene.
• Their function in breast cancer: A HER2 gene mutation in the breast cells can cause too many HER2 receptors to be created in breast cells, signaling the cells to grow and divide out of control.
• Biomarker test used: An IHC test, a fluorescence in situ hybridization (FISH) test, or both. These are different staining techniques done on a sample of cancer tissue.
• How a positive result guides treatment: HER2-positive breast cancers can be treated with HER2-targeted therapies such as trastuzumab (Herceptin). MBC with low amounts of HER2 receptors, known as HER2-low breast cancer, may be eligible for treatment with trastuzumab-deruxtecan (Enhertu).

Learn more about HER2-positive or HER2-low breast cancer.

Genomic tests such as EndoPredict, MammaPrint, OncotypeDX, and Prosigna look at the expression of groups of genes to predict the risk of recurrence in 10 years. Gene expression is a way to describe whether a gene in a cell is “turned on” or “turned off.” These genomic tests help guide decisions about whether chemotherapy is needed to reduce the risk of recurrence. The Breast Cancer Index test is another genomic test that looks at gene expression to predict the risk of hormone receptor-positive cancer recurring from five to 10 years after diagnosis. This can determine whether you are likely to benefit from continuing endocrine therapy beyond five years.

Genomic tests include any test that looks at genes inside of cancer cells. These are not the same as genetic tests for inherited mutations, which look for mutations such as BRCA1 or BRCA2 that are passed down by a family member. You can learn more about inherited mutations in the Genetics and family risk section.

• What it is: A gene mutation in breast cells that gives instructions for making the AKT1 protein, which helps regulate cell division, function, and survival.
• Its function in breast cancer: The AKT1 mutation can abnormally activate the AKT1 protein, allowing cells to grow and divide without control.
• Biomarker test used: A test such as FoundationOne CDX that uses next-generation sequencing technology to look for gene mutations in cancer cells. NGS can be used on tumor tissue or a blood sample. These tests can search hundreds of genes at the same time for mutations and alterations.
• How a positive result guides treatment choices: Breast cancer that is hormone receptor-positive, HER2-negative, and tests positive for an AKT1 mutation can be treated with capivasertib (Truqap) along with the endocrine therapy fulvestrant.

• What it is: A mutation in the estrogen-receptor 1 gene, abbreviated as ESR1. The ESR1 gene carries instructions for building estrogen receptors on the surface of cells.
• Its function in breast cancer: In people with hormone receptor-positive, HER2-negative breast cancers, an ESR1 mutation indicates the cancer is no longer as responsive to endocrine therapy, especially aromatase inhibitors. This mutation changes the shape of the estrogen receptors. As a result, the receptors become activated, like an “on” switch that stays on, and tell the cancer cells to keep growing.
• Biomarker test used: A test that uses NGS, such as Guardant 360 CDx or FoundationOne CDx. These tests search for multiple gene mutations at the same time.
• How a positive result guides treatment choices: The cancer may be eligible for treatment with elacestrant (Orserdu), a newer form of endocrine therapy, depending on what treatments a person had in the past.

Learn more about ESR1 mutation testing.

• What it is: PD-L1 is a protein found on healthy cells that prevents cells in the immune system, called T cells, from attacking them.
• Its function in breast cancer: If cancer cells have too much PD-L1 on their surface, they can turn the body’s T cells off so that they won’t attack the cancer. This allows breast cancer to grow unchecked.
• Biomarker test used: An IHC test uses a sample of cancer tissue to measure the amount of PD-L1 present.
• How a positive result guides treatment choices: In metastatic, triple-negative breast cancer (which tests negative for hormone receptors and HER2 receptors), a positive PD-L1 result means the cancer can be treated with pembrolizumab (Keytruda), an immunotherapy that blocks PD-L1 to help the body’s own immune system to attack cancer cells.

• What it is: A mutation in the PIK3CA gene that makes one of the proteins in an enzyme called PI3K, which is important in many cell functions.
• Its function in breast cancer: In MBC cells, a PIK3CA mutation can cause PI3K to become too active, allowing cancer cells to grow and thrive.
• Biomarker test used: A test that uses NGS technology, such as FoundationOne CDx, can look for PIK3CA mutations using a tissue or blood sample.
• How a positive result guides treatment: MBC that is hormone receptor-positive, HER2-negative, and PIK3CA mutation-positive can be treated with the targeted therapy alpelisib (Piqray) along with the endocrine therapy fulvestrant. Another option is capivasertib plus fulvestrant.

• What it is: An alteration in the PTEN gene, which provides instructions for making an enzyme that keeps cells from dividing and growing out of control.
• Its function in breast cancer: Alterations in the PTEN gene can cause uncontrolled cell growth and division, leading to the development of breast cancer. In some cases, the mutation is inherited from a parent, but more commonly, it develops in breast cells.
• Biomarker test used: A test that uses NGS technology on blood or tumor tissue, such as FoundationOne CDx or Guardant 360 CDx.
• How a positive result guides treatment: MBC that is hormone receptor-positive, HER2-negative, and PTEN-alteration positive can be treated with capivasertib plus fulvestrant. Capivasertib interrupts the activity of PTEN.

• What it is: A mutation that happens when the NTRK gene fuses, or joins with, a gene on another chromosome.
• Its function in breast cancer: NTRK gene fusion can produce abnormal proteins that may cause cancer cells to grow.
• Biomarker test used: FISH tests and tests that use NGS technology, such as FoundationOne CDx and Guardant 360.
• How a positive result guides treatment: Locally advanced breast cancer or MBC testing positive for NTRK gene fusion may be eligible for treatment with larotrectinib (Vitrakvi), or entrectinib (Rozlytrek), targeted therapies that block the abnormal proteins from helping cancer cells grow.

• What it is: MSI-H stands for “microsatellite instability-high,” which means that breast cancer cells have a high number of mutations in short, repeated sequences of DNA (known as microsatellites).
• Its function in breast cancer: MSI-H breast cancer cells may be less able to correct DNA mistakes. This is sometimes called mismatch repair deficient (dMMR).
• Biomarker tests used: IHC tests, tests that use NGS technology such as FoundationOne CDx, or PCR (polymerase chain reaction) tests, which use a sample of blood, cancer tissue, or saliva to look for specific gene changes.
• How a positive result guides treatment: MSI-H breast cancer may be eligible for treatment with immunotherapies such as pembrolizumab or dostarlimab (Jemperli).

• What it is: TMB stands for tumor mutational burden. It is a measure of how many mutations are in the DNA of cancer cells.
• Its function in breast cancer: High TMB appears to be associated with higher production of abnormal tumor proteins. This means that immunotherapy may be more likely to be effective.
• Biomarker test used: A somatic mutation test, such as FoundationOne CDx, that uses NGS technology on tumor tissue or a blood sample to look at hundreds of genes at once.
• How a positive result guides treatment: TMB-High breast cancer may be eligible for treatment with the immunotherapy pembrolizumab.

• What it is: RET fusion happens in cancer cells when the DNA of a gene called RET fuses with a different gene. This activates an enzyme that tells cells to keep growing, leading to tumor formation.
• Its function in breast cancer: RET fusion can drive uncontrolled cell growth.
• Biomarker test used: A test that uses NGS technology on tumor tissue or a blood sample, such as FoundationOne CDx. NGS looks at hundreds of genes at once.
• How a positive result guides treatment: MBC with RET fusion may be eligible for treatment with selpercatinib (Retevmo), which targets the enzyme that causes the cancer cells to grow.

Cancer antigens are proteins or other molecules found on the surface of cancer cells, but not normal cells. In MBC, if certain antigen levels are elevated at the time of diagnosis, they may be used to follow the response to treatment. These tests also may be useful if your doctor suspects metastases to the bones or liver.

If the antigen level increases over time, your doctor may decide to switch therapies or monitor you more closely. However, the tests aren’t perfect, because antigen levels can be elevated for other reasons besides cancer. Examples of antigens include:

• CA 15-3: These biomarkers may be used to monitor MBC during treatment.
• CA 27-29: These biomarkers also may be used to monitor MBC during treatment.
• CEA: CEA stands for carcinoembryonic antigen. High levels may indicate that breast cancer has spread to other parts of the body. Testing for CEA can also help doctors monitor how the cancer is responding to treatment.

Circulating tumor cells are cancer cells that have broken away from a tumor and entered the blood. CTC blood tests can be used to look for early signs of recurrence and monitor response to treatment. Tests such as CellSearch can detect CTCs in the blood.

Biomarker testing is testing for hormone receptors, HER2 receptors, gene expression, genetic mutations, proteins, or other substances that can be found in the cancer tissue or in the blood. These tests can provide more information about the biology of the cancer: what’s driving the cancer’s growth, how likely it is for early-stage breast cancer to come back, or whether MBC is progressing. This information helps guide treatment choices.

Doctors can use biomarker test results to select specific treatments that are more likely to be effective against the cancer. These results also can be helpful in predicting the risk of recurrence, and how likely the cancer is to grow and spread. A higher-risk breast cancer may require closer follow-up.

No, biomarker testing cannot predict treatment side effects. However, it can often be helpful in selecting targeted therapies, which often have less intense side effects than chemotherapy.

The most common biomarkers tested for in breast cancer are:

• Hormone receptors, to find out if the cancer is hormone receptor-positive or negative
• HER2 receptors, to find out if the cancer is HER2-positive, HER2-low, or HER2-negative

These tests are done after every breast cancer diagnosis and can also be done again later, after lumpectomy, mastectomy, or if cancer recurs or metastasizes. Other common breast cancer biomarkers include:

• Gene expression in early-stage, hormone receptor-positive tumor tissue seen in genomic tests such as Oncotype DX and MammaPrint; looking at certain groups of genes to learn whether they are “turned on” or “turned off” can determine the likelihood of recurrence and whether chemotherapy is likely to be beneficial
• Gene mutations in locally advanced breast cancer or MBC tissue, such as mutations in ESR1, PIK3CA, or AKT1 genes, which can help match specific breast cancers with specific endocrine or targeted therapies
• PD-L1 proteins on MBC cells, which can help match specific breast cancers with immunotherapies