Breast Cancer Biomarkers
What Patients Need to Know
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
Why This Matters
Your doctor is looking for clues about your cancer
When breast cancer is found, doctors test the tumor for special markers — signals that tell them exactly what kind of cancer you have and which medicines are most likely to help you.
Think of it like a lock and key. Biomarker tests find the lock on your cancer. Then your doctor can find the right key — the treatment that matches.
The Biomarkers Your Doctor Tests
🔍 Does your cancer use estrogen to grow?
If yes (ER+), hormone-blocking pills like tamoxifen can starve the cancer of what it needs to grow. This is one of the most effective breast cancer treatments available.
🔍 Does your cancer use progesterone to grow?
Similar to ER. If your tumor is PR+, hormone therapy is likely to help. PR is almost always tested together with ER.
🔍 Does your cancer have too much of a growth-driving protein?
HER2+ tumors grow faster — but there’s good news. Drugs like Herceptin (trastuzumab) are specifically designed to block HER2 and work very well.
🔍 How fast is your cancer growing?
Ki-67 measures how many cancer cells are actively dividing. A higher number means faster growth. This helps your doctor decide how aggressive your treatment needs to be.
🔍 Did you inherit a gene mutation that increases cancer risk?
If you have a BRCA mutation, certain drugs called PARP inhibitors (like olaparib) may work well for you. This result also matters for your family members — they may want to get tested too.
Sources: Susan G. Komen · NCI · ARUP Consult
Common Questions
Breast Cancer Biomarkers — Frequently Asked Questions
❓ What does ER-positive breast cancer mean?
It means your tumor feeds on estrogen to grow. The good news: hormone-blocking medicines like tamoxifen can cut off that fuel supply. ER-positive breast cancer is the most common type and often has very good treatment options.
❓ What is HER2-positive breast cancer?
HER2-positive means your tumor has too much of a protein that makes cancer cells grow fast. Targeted drugs like Herceptin are designed to block that protein — and they work very well. Being HER2-positive is no longer considered the bad news it once was.
❓ What is triple-negative breast cancer?
Triple-negative means your cancer is ER-negative, PR-negative, and HER2-negative — so hormone therapy and HER2 drugs won’t work. Chemotherapy and immunotherapy are the main options. Ask your doctor about clinical trials, as this is an active area of research.
❓ Should I get BRCA testing?
Yes, 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 opens the door to PARP inhibitor treatment and is important information for your close relatives.
Know Your Biomarkers. Own Your Journey.
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Visit the Breast Cancer Center → Full Biomarkers Guide →
