Biomarker Resource Center
Breast Cancer Biomarkers
Reviewed by XpertPatient Medical Advisory · Updated August 2026 · Primary Source: cancer.gov
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What Is a Biomarker — and Why Does It Matter for Your Treatment?
A biomarker is a signal your cancer leaves behind — found in your blood or tumor tissue. These signals tell your doctor what is driving your specific cancer to grow.
Think of it like a clue. Your doctor collects clues from your biomarker test results. Those clues help build a picture of your cancer — and that picture helps you have a more informed conversation with your care team about next steps.
How a Biomarker Test Works
A biomarker is a signal your cancer leaves behind. Your doctor finds it through a biopsy or blood test, reads your results, and uses them to choose the treatment most likely to work for your specific tumor.
Why Do Biomarkers Matter in Breast Cancer?
Two people can both have breast cancer but need completely different treatments. Biomarker tests help your doctor understand what is fueling your specific tumor — and which treatment approaches are most likely to work for you.
The most important breast cancer biomarkers are ER, PR, and HER2. Together they help your doctor choose the treatment approach most likely to work for your tumor.
Ask your doctor: Can you walk me through my ER, PR, and HER2 results and what they mean for my care plan?
Quick Poll
Have you been biomarker tested?
ER, PR, and HER2 status is the single biggest factor in matching you to the right treatment.
Source: IQVIA Institute, Advancing Precision Oncology, March 2026 · established biomarker testing rate in metastatic breast cancer · iqviainstitute.org
The Tests Your Doctor May Order
Does your cancer use estrogen to grow?
An ER-positive result means estrogen may be helping the cancer grow. Your doctor will talk with you about approaches that work by reducing estrogen’s effect. About 70% of breast cancers are ER-positive. Ask your doctor what your ER result means for your care.
Does your cancer use progesterone to grow?
Similar to ER. If PR-positive, your doctor will consider this alongside your ER result. ER and PR are almost always tested together. Ask your doctor what both results mean for your care plan.
Does your cancer make too much of a growth protein?
HER2-positive means tumor cells have too many HER2 proteins, causing cancer cells to multiply faster than normal. About 15% of breast cancers are HER2-positive. There are approaches specifically for HER2-positive breast cancer. Ask your doctor what HER2-positive means for your situation.
How fast is your cancer growing?
Ki-67 measures how many cancer cells are actively dividing right now. A higher number means faster growth. Your doctor uses this alongside your other results to understand your tumor. Ask your doctor what your Ki-67 number means for your care plan.
Did you inherit a gene that may affect your care?
BRCA testing is done from a blood draw — it looks at inherited genes, not your tumor. A BRCA mutation may affect your care options and matters for close family members too. Ask your doctor whether BRCA testing makes sense for you.
Sources: National Cancer Institute · Susan G. Komen · American Cancer Society
Watch & Learn
Common Questions
Breast Cancer Biomarkers — Your Questions Answered
What does ER-positive mean?
ER-positive means your tumor may use estrogen to grow. Your doctor will discuss approaches that work by reducing estrogen levels. Ask your doctor what ER-positive means for your specific care plan.
What is HER2-positive breast cancer?
HER2-positive means your tumor makes too much of a protein that speeds up cancer cell growth. There are approaches designed to block this protein. Ask your doctor what HER2-positive means for your situation.
What is triple-negative breast cancer?
Triple-negative means ER-negative, PR-negative, and HER2-negative. Hormone therapy and HER2-targeted approaches are not options. Your care team will discuss other approaches. Ask your doctor about all options including clinical trials.
Should I ask about BRCA testing?
BRCA testing is commonly recommended if you were diagnosed at a younger age, have a family history, or have triple-negative breast cancer. Ask your doctor whether it makes sense for your situation.
Knowledge Check
Test what you’ve learned
Three quick questions to help you feel more confident talking with your care team.
What does ER-positive mean?
Based on what you just read, which best describes an ER-positive result?
Which test tells your doctor how fast your cancer cells are dividing?
Based on what you just read, which biomarker measures how quickly cancer cells multiply?
What does triple-negative breast cancer mean?
Based on what you just read, which best describes a TNBC diagnosis?
Keep these questions in mind for your next appointment with your care team.
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