Evidence-Based Guide to Biomarkers for Breast Cancer

When people hear "breast cancer biomarkers," they often think there might be a single blood test that can find or forecast breast cancer. That's not how this works. In breast cancer, biomarkers are a set of lab measurements on tumor tissue and sometimes blood that help define the biology of the cancer, estimate risk, and match treatment. Mammograms and MRIs are for screening. Biomarkers guide decisions after a diagnosis, and sometimes help monitor disease later. This guide walks you through what matters, what the major guidelines recommend, and what those results really mean.

What a biomarker does in breast cancer

Biomarkers fall into four practical buckets: diagnostic (classify the tumor), prognostic (estimate the chance of recurrence), predictive (estimate the chance a specific treatment will help), and monitoring (track disease activity over time). A good way to think about it is like tuning a fitness plan to your own physiology. The diagnosis is the starting point, but biomarkers personalize the plan, making sure the intensity matches your body's response.

The three core biomarkers every invasive breast cancer is tested for

Every invasive breast cancer should be tested for estrogen receptor (ER), progesterone receptor (PR), and HER2. This isn't optional—major guidelines (ASCO/CAP, NCCN, ESMO) treat these as the backbone of care because they predict benefit from therapies that can be life-saving.¹

ER and PR: Hormone receptors

What they are: Proteins on or in tumor cells that bind estrogen or progesterone. If present, the tumor is described as hormone receptor–positive (HR+).

How they're measured: Immunohistochemistry (IHC) on tumor tissue. Results are reported as a percentage of tumor cells staining and intensity. Even low-level ER positivity can matter, but most benefit is seen when a substantial fraction of cells express ER.

Why they matter: ER and PR positivity predicts tumor response to endocrine therapy, which lowers hormone signaling. That can be pills, injections, or ovarian suppression in premenopausal patients. It also gives prognostic information; HR+ cancers tend to have a longer natural history with late recurrences, which is why discussions about the duration of endocrine therapy exist.

HER2: A growth signaling receptor

What it is: A receptor that, when overexpressed or amplified, drives tumor growth. HER2 status divides breast cancer into two biologically distinct groups.

How it's measured: IHC for protein overexpression and, when equivocal, in situ hybridization (ISH) for gene amplification. Pathology reports use standardized scoring (0 to 3+). HER2-positive means IHC 3+ or ISH-amplified; HER2-negative means 0 or 1+, or 2+ with negative ISH.²

Why it matters: HER2-positive disease is treated with HER2-targeted therapies and has improved dramatically with these drugs. A newer nuance is "HER2-low" (IHC 1+ or 2+ with negative ISH). While not "positive" in the classic sense, this category can be relevant for specific therapies in metastatic settings.²

Limitations: Interpretation depends on rigorous technique and experienced pathology. Bone biopsies that underwent decalcification may yield unreliable HER2 results.³ As with ER/PR, HER2 status can evolve—re-biopsy can correct course if results shift.

Ki-67 and proliferative markers: Reading the speedometer

Ki-67 measures how quickly tumor cells are dividing. Higher values often correlate with more aggressive biology. In some specific settings, Ki-67 has helped identify which early HR+ patients have higher risk biology.⁴ However, assays vary, and scoring can be inconsistent across labs. Most guidelines view Ki-67 as supportive information rather than a stand-alone decision-maker.⁴ It can add color to the picture but rarely paints the whole canvas by itself.

Genomic assays for early-stage HR+/HER2− cancer: Who benefits from chemotherapy?

For many with early-stage, node-negative or limited node-positive HR+/HER2− breast cancer, the critical question is whether chemotherapy adds meaningful benefit on top of endocrine therapy. Multigene assays analyze patterns of tumor gene expression to estimate recurrence risk and potential chemotherapy benefit. They don't "diagnose" cancer—they refine risk and personalize therapy intensity.

Oncotype DX (Recurrence Score)

What it provides: A numerical score based on 21 genes that estimates 10-year distant recurrence risk and predicts chemotherapy benefit in HR+/HER2− early breast cancer.

Evidence: TAILORx and RxPONDER showed that many postmenopausal patients with low to intermediate scores can avoid chemotherapy without compromising outcomes, while specific groups, especially younger or premenopausal patients, can still benefit at similar scores.⁵ ⁶ That's a key nuance—menopausal status matters because part of chemotherapy's impact in younger patients relates to ovarian function suppression.⁶

How it's used: Routinely considered for stage I–II HR+/HER2− cancers and sometimes limited node-positive disease to guide adjuvant therapy planning.

Limitations: Not validated for HER2-positive or triple-negative cancers; results may be less informative if the tumor has peculiar histology or if tissue processing was suboptimal.

MammaPrint (70-gene), Prosigna (PAM50), EndoPredict, and Breast Cancer Index

These assays share a purpose—refine risk and, in some cases, estimate the chance of late recurrence beyond five years. MammaPrint (MINDACT trial) helped identify patients with high clinical, low genomic risk who could safely forgo chemotherapy.⁷ Prosigna and EndoPredict add risk grouping that can stratify late recurrence risk.⁸ Breast Cancer Index focuses on two questions: risk of recurrence in years 5–10 and whether extended endocrine therapy might help. Each has its niche and evidence base; choice depends on clinical context, local availability, and guideline alignment.

Limitations: Assay outputs are not interchangeable. A "low risk" from one test does not automatically equal "low risk" from another. Results inform decisions but don't replace clinical judgment or patient preferences.

Germline genetic testing: Inherited risk and actionability

Germline testing looks for inherited variants that raise breast cancer risk and can change care for you and your family. It's different from tumor testing; this is DNA you were born with, typically from blood or saliva.

Who commonly meets criteria

Guidelines typically recommend testing for anyone with early-onset breast cancer, triple-negative disease diagnosed under age 60, male breast cancer, multiple relatives with breast or ovarian cancer, or specific ancestries with higher prevalence of founder variants. Many centers now use broader criteria because the results can change both treatment and prevention strategies.⁹

Key genes and why they matter

• BRCA1 and BRCA2: Higher lifetime risks of breast and ovarian cancer; inform screening intensity, risk-reducing surgery discussions, and sensitivity to specific therapies like PARP inhibitors.
• PALB2: Risk nearly comparable to BRCA2 in some studies; affects screening and surgical planning.
• CHEK2 and ATM: Moderate risk genes that can shift screening strategies and inform family testing.
• TP53 (Li-Fraumeni syndrome): High risk of multiple cancers; impacts imaging choices due to radiation sensitivity and prompts tailored surveillance.

Limitations: A "variant of uncertain significance" is not actionable. Genetic counseling helps interpret results and plan next steps. A negative result in one person doesn't rule out risk in the family without testing an affected relative when possible.

Triple-negative breast cancer (TNBC): Immune and emerging biomarkers

TNBC lacks ER, PR, and HER2 amplification. Biology varies widely, which is why additional biomarkers can help.

PD-L1 expression

In metastatic TNBC, PD-L1 testing on tumor tissue can identify who is more likely to benefit from immunotherapy.¹⁰ Different assays and scoring systems exist (combined positive score vs. immune cell scoring), and not all are interchangeable. Early-stage immunotherapy decisions generally don't rely on PD-L1, but metastatic options often do.

Tumor-infiltrating lymphocytes (TILs)

TILs reflect the immune system's presence in the tumor. Higher TILs are associated with better outcomes in TNBC.¹¹ While promising and prognostic, TILs are not widely used to change therapy on their own in routine practice.

Metastatic disease: Biomarkers for precision targeting

When breast cancer spreads, the biology can evolve. Re-biopsy of a metastatic site, when feasible, helps confirm current ER, PR, and HER2 status. Beyond the basics, several actionable tumor alterations matter.

PIK3CA mutations

What they are: Mutations that activate the PI3K pathway in a significant fraction of HR+/HER2− metastatic cancers.

Why they matter: Presence of a PIK3CA mutation predicts benefit from a PI3K inhibitor combined with endocrine therapy. Testing can be done on tumor tissue or circulating tumor DNA (ctDNA); a positive result in ctDNA is informative, while a negative result can be a false negative if the tumor isn't shedding detectable DNA.¹² ¹³

ESR1 mutations

What they are: Changes in the estrogen receptor gene that can drive resistance to aromatase inhibitors.

Why they matter: ESR1 mutations can be detected by ctDNA and may inform switching endocrine strategies, including consideration of agents that directly target or degrade the receptor.¹⁴

HER2 mutations

Even when HER2 is not amplified, some tumors harbor activating HER2 mutations. These can predict sensitivity to specific HER2-targeted tyrosine kinase inhibitors in metastatic settings.¹⁵ Tumor sequencing or ctDNA can identify them.

BRCA and other homologous recombination repair genes

Germline BRCA1/2 variants and, in some cases, somatic alterations in DNA repair pathways can inform sensitivity to PARP inhibition in metastatic disease. This underscores why pairing germline and tumor testing can be complementary.

Rare but actionable fusions and high mutational burden

NTRK fusions are rare in breast cancer but, if present, open a path to targeted therapy.¹⁶ Tumor mutational burden and MSI-high status are uncommon here; when found, they may qualify for tumor-agnostic immunotherapy.

Liquid biopsy and ctDNA: What's ready, what's experimental

Liquid biopsy analyzes tumor DNA fragments circulating in the bloodstream. It's useful for detecting specific mutations like PIK3CA or ESR1 in metastatic disease, particularly when a tissue biopsy is hard to obtain.¹³ It can also help track the emergence of resistance mutations over time.

Serum tumor markers: CA 15-3, CA 27.29, and CEA

These are blood tests measuring proteins shed by tumor cells. They're not for screening or diagnosis. Guidelines caution against using them to detect cancer in people without symptoms because benign conditions, liver disease, and even pregnancy can elevate levels.¹⁷ In metastatic breast cancer, clinicians sometimes track these markers to follow trends in response to therapy. Rising markers alone shouldn't trigger a change in treatment without imaging or clinical correlation.¹⁷ Assays vary, and fluctuations can occur for reasons unrelated to tumor growth.

Breast density and risk models: Not treatment biomarkers, but useful context

Breast density is a radiologic feature that increases both cancer risk and the chance that a mammogram can miss a lesion.¹⁸ It's not a tumor biomarker, but it influences screening strategy. Risk models (like Tyrer–Cuzick) integrate age, family history, reproductive factors, and sometimes polygenic risk scores to estimate lifetime risk. These tools guide imaging choices such as adding MRI for higher-risk individuals. Polygenic risk scoring is promising yet still integrating into routine care—results should be used alongside, not instead of, clinical factors.

Life-stage and sex differences that influence biomarker use

• Premenopausal vs. postmenopausal: Genomic assays may yield similar numerical scores, but chemotherapy benefit can differ by menopausal status. Younger patients may gain benefit that reflects both cytotoxic effects and ovarian suppression. This is a core finding from large trials and shows up in guideline algorithms.⁶
• Pregnancy: Serum tumor markers can physiologically rise, so they're not reliable. Imaging choices and timing of treatment are tailored; biomarker interpretation relies heavily on tissue testing rather than blood markers during pregnancy.
• Male breast cancer: More likely to be ER-positive. Germline testing yields actionable findings more often than in unselected female populations, so criteria for genetic evaluation are broad for men with breast cancer.

How the tests are done: Practicalities that affect accuracy

Pathology and laboratory science details can change results. Good fixation of the surgical specimen, minimizing cold ischemia time, and using validated antibody clones and scoring systems are essential for accurate ER/PR/HER2 and Ki-67 results.¹ For HER2, IHC 2+ triggers ISH testing to clarify amplification. For PD-L1, the assay used matters because different drugs validated different tests with distinct cutoffs.

For tumor sequencing, whether tissue was recently obtained, if the sample has enough tumor content, and whether decalcification was used, all affect detectability of mutations.³ ctDNA tests need careful handling and rapid processing to preserve DNA fragments; a negative ctDNA result doesn't exclude a mutation if the tumor isn't shedding detectable DNA at that moment.

Putting it together: A typical pathway

At diagnosis after a core needle biopsy, the pathology report should include histologic type, grade, and ER/PR/HER2. If surgery is first, final pathology refines those findings and confirms margins and nodal status. For HR+/HER2− early-stage disease, a genomic assay is often ordered to clarify the role of chemotherapy. If nodes are positive or disease is more advanced, the plan may include systemic therapy first, with biomarkers guiding choices and sequence.

During follow-up after curative treatment, routine imaging or blood tumor markers aren't recommended for asymptomatic patients; evidence shows they don't improve survival and can cause anxiety and false alarms. If cancer recurs or becomes metastatic, re-biopsy confirms current ER/PR/HER2. Tumor sequencing or ctDNA can look for PIK3CA, ESR1, HER2 mutations, or rare fusions, which helps align therapy with the tumor's current behavior. Serum tumor markers, if used, track alongside scans and symptoms rather than replace them.

What results mean in real life

Biomarkers don't make decisions by themselves—people do. A patient with a low genomic risk score and a high-intensity career might prioritize avoiding chemotherapy's side effects and accept a very small increase in recurrence risk. Another might choose maximum therapy for peace of mind despite a low score. Biomarkers are guideposts. They improve the signal-to-noise ratio so decisions can be matched to biology and values.

Common misunderstandings, clarified

• "Is there a blood test to find breast cancer?" No. Mammography and MRI are screening tools. Serum tumor markers are not used to detect cancer in people without symptoms.
• "If my tumor marker is up, does that mean the cancer is growing?" Not necessarily. Infections, liver inflammation, and lab variation can nudge markers. Trends plus imaging are the gold standard for assessment.
• "My HER2 was negative the first time—does that ever change?" Yes. Tumor biology can evolve. Re-biopsy can uncover changes that open doors to new treatments.
• "If my genetic test is negative, does that mean my kids aren't at risk?" It depends. If no affected relative was tested, a negative in you may be less informative. Genetic counseling helps interpret what the result means for family members.

A note on quality, access, and equity

High-quality biomarker testing relies on standardized lab procedures and access to multidisciplinary care. Not every lab uses the same assays, and insurance coverage can be uneven for newer tests. If results seem out of step with your clinical picture—a HER2-negative result in a tumor behaving like HER2-positive, for example—asking the team about repeat testing or a second review is appropriate. This is not about second-guessing; it's about aligning care with accurate biology.

Credibility cues you can trust

• ASCO/CAP guidelines set standardized methods for ER/PR/HER2 and emphasize re-testing if disease biology changes.¹
• TAILORx and RxPONDER established how multigene assays can safely reduce chemotherapy use in many HR+/HER2− patients while preserving outcomes.⁵ ⁶
• MINDACT confirmed genomic risk can reclassify many clinically high-risk patients as candidates to avoid chemotherapy.⁷
• Multiple guideline groups caution against tumor markers for screening and advise using them selectively for monitoring metastatic disease.¹⁷

Bottom line

Biomarkers for breast cancer are not a single test; they're a toolkit. ER, PR, and HER2 define the broad strategy. Genomic assays in early HR+/HER2− disease refine who benefits from chemotherapy. Germline testing safeguards families and opens targeted options. In metastatic disease, tumor sequencing and ctDNA help track resistance and unlock precision therapies. Each test has strengths and limitations, and results must be interpreted in context. When used thoughtfully, biomarkers turn a one-size-fits-all diagnosis into a tailored plan that fits the biology of the cancer and the priorities of the person living with it.