What Are Cancer Biomarkers? HER2, EGFR, BRCA and More, Explained

If you have cancer, you may hear your care team talk about "biomarker testing." The names on the report can look like alphabet soup. Here's a plain-language guide to what they mean and why they matter.

What is a biomarker?

Biomarkers are genes, proteins or other substances that can be tested to learn more about your cancer.² Some treatments, including certain targeted therapies and immunotherapies, may only work for people whose cancers have certain biomarkers.¹

Biomarker testing is done routinely to choose treatment for some cancers, including non-small cell lung, breast and colorectal cancer.¹ You may also hear it called tumor testing, genomic profiling or molecular testing.¹

How the test is done

For a solid tumor, a sample is usually taken during surgery or a biopsy. For blood cancers, or for a "liquid biopsy," a blood draw is used.¹ Biomarkers can change over time, so your doctor may test your cancer again, for example if it comes back.¹

Common biomarkers, explained

• HER2. HER2 is a protein that some cancer drugs target. One example is a drug that links an antibody aimed at HER2 to a chemo drug.³ In 2025, the FDA approved the drug fam-trastuzumab deruxtecan for some advanced breast cancers with low levels of HER2, called "HER2-low" or "HER2-ultralow."⁴
• EGFR. EGFR is a protein on the surface of cells that normally helps them grow and divide.⁵ Drugs called EGFR inhibitors are often used for advanced non-small cell lung cancers with certain EGFR gene mutations.⁵
• KRAS. KRAS is one of the main RAS genes.⁶ Changes in RAS genes drive about a third of all human cancers, including a high percentage of pancreatic, lung and colorectal cancers.⁶ For lung cancer with a KRAS G12C change, targeted drugs include sotorasib and adagrasib.⁵
• BRCA1 and BRCA2. These genes make proteins that help repair damaged DNA.⁷ An inherited harmful change raises the risk of breast and ovarian cancer, and also prostate and pancreatic cancer.⁷ Some cancers with these changes can be treated with drugs called PARP inhibitors.⁷
• PD-L1. Some tumors make large amounts of PD-L1, a protein that keeps T cells from killing tumor cells. Drugs called checkpoint inhibitors block this "off" signal.⁸
• DNA repair problems. Some solid tumors can't repair errors in their DNA that happen when the DNA is copied. Checkpoint inhibitors are approved for some people with any solid tumor of this kind.⁸
• TMB. This stands for tumor mutational burden, the number of genetic changes in your cancer.¹ The FDA approved the checkpoint inhibitor pembrolizumab for some advanced solid tumors with high TMB (10 or more mutations per megabase) that have grown after earlier treatment and have no good alternatives.⁹

Tumor testing vs. genetic testing

Biomarker testing looks for changes that came about in the cancer during your lifetime. Genetic testing for cancer risk looks for inherited changes you were born with.¹

Genetic testing for inherited risk usually uses blood, but it can also use saliva or cells from inside your cheek.¹⁰ Some biomarker tests can find changes you may have been born with, which may lead to more genetic testing and counseling.¹ Genetic counseling is generally recommended before testing for inherited cancer risk.¹⁰

What results can and can't tell you

Biomarker tests don't help everyone who gets them. A matching treatment may not exist, or it may not work for every person.¹ Testing can also help you find a clinical trial of a new treatment.¹

Good questions to ask: "Can my cancer be tested for biomarkers? Why or why not?" and "If my cancer has a biomarker, what does that mean for my treatment?"² Ask about cost, too. Medicare and Medicaid cover some tests for advanced cancer, and private insurers often cover a test when there's enough proof it's needed to guide treatment decisions.¹

Sources

1. Biomarker Testing for Cancer Treatment, National Cancer Institute
2. Cancer Biomarkers (patient booklet), American Cancer Society
3. Monoclonal Antibodies and Their Side Effects, American Cancer Society
4. FDA approves fam-trastuzumab deruxtecan-nxki for unresectable or metastatic HR-positive, HER2-low or HER2-ultralow breast cancer, U.S. Food and Drug Administration
5. Targeted Drug Therapy for Non-small Cell Lung Cancer, American Cancer Society
6. The RAS Initiative, NCI at Frederick
7. BRCA Gene Changes: Cancer Risk and Genetic Testing, National Cancer Institute
8. Immune Checkpoint Inhibitors, National Cancer Institute
9. FDA approves pembrolizumab for adults and children with TMB-H solid tumors, U.S. Food and Drug Administration
10. Genetic Testing for Inherited Cancer Risk, National Cancer Institute

For education only, not medical advice. Biomarkers, approved treatments and testing guidelines change often, so always talk with your oncology team about your own care.