Breast Cancer Biomarkers
The most important breast cancer biomarkers are ER, PR, and HER2 — together they determine whether hormone therapy or targeted drugs like Herceptin are likely to work for your tumor.
Reviewed by XpertPatient Medical Advisory · Updated June 2026 · Sources: NCI · ACS · ASCO · NIH/PMC
Start Here
What is a biomarker?
A biomarker is a measurable signal found in your blood or tumor tissue. In breast cancer, biomarker tests tell your doctor exactly what is driving your cancer to grow — and which medicines can stop it.
Breast cancer is not one disease. Two women with the same stage breast cancer can have completely different tumors — and need completely different treatments. Biomarker tests are how your doctor figures out which treatment is right for you specifically.
What this means for you: The first question your doctor answers with biomarker testing is: what is fueling your cancer’s growth? The answer determines your entire treatment plan.
The Tests Your Doctor Orders
🔍 Does your cancer use estrogen to grow?
If yes — called ER-positive — hormone-blocking pills like tamoxifen can cut off the fuel your cancer needs. ER-positive is the most common breast cancer type and often has excellent treatment options.
🔍 Does your cancer use progesterone to grow?
Similar to ER. If PR-positive, hormone therapy is likely to help. PR is almost always tested together with ER.
🔍 Does your cancer make too much of a growth-driving protein?
HER2-positive tumors grow faster — but targeted drugs like Herceptin (trastuzumab) block that protein directly and work very well. Being HER2-positive is no longer the difficult diagnosis it once was.
🔍 How fast is your cancer growing?
Ki-67 measures how many cancer cells are dividing right now. A higher number means faster growth. This helps your doctor decide how intensive your treatment needs to be.
🔍 Did you inherit a gene mutation that affects treatment?
A BRCA mutation means certain drugs called PARP inhibitors may work well for you. It also matters for your family — close relatives may want to get tested too.
Sources: Susan G. Komen · NCI · ARUP Consult
Watch & Learn
Common Questions
Breast Cancer Biomarkers — Your Questions Answered
What does ER-positive mean?
It means your tumor feeds on estrogen to grow. Hormone-blocking medicines like tamoxifen cut off that fuel. ER-positive is the most common breast cancer type and often responds very well to treatment.
What is HER2-positive breast cancer?
HER2-positive means your tumor makes too much of a protein that speeds up cancer cell growth. Drugs like Herceptin are designed to block that protein — and they work very well. A HER2-positive result today comes with good targeted treatment options.
What is triple-negative breast cancer?
Triple-negative means ER-negative, PR-negative, and HER2-negative. Hormone therapy and HER2 drugs won’t work, so chemotherapy and immunotherapy are the main options. Ask your doctor about clinical trials — this is an active research area.
Should I get BRCA testing?
Yes — especially if you have triple-negative breast cancer, were diagnosed under 50, or have a family history of breast or ovarian cancer. A positive BRCA result can open the door to additional treatment options.
Know Your Biomarkers. Own Your Journey.
Free, survivor-led cancer education for newly diagnosed patients.
Visit the Breast Cancer Center → Full Biomarkers Guide →
