IHC is a common method used in the diagnosis of breast cancer and research into breast cancer pathology. These markers are used to determine different breast cancer types, eg, in situ or invasive carcinoma, distinguishing normal breast cell types, eg, luminal, basal, and myoepithelial, and also proliferation and disease progression.

In addition to tissue-based markers, tumor markers are proteins or substances measured in blood tests to monitor breast cancer progression, treatment response, and recurrence. Common tumor markers include CA15-3, CA27.29, and carcinoembryonic antigen (CEA). These tumor marker tests are blood tests performed via blood draw, and while they are not used to diagnose breast cancer directly, they are helpful in monitoring advanced breast cancer and guiding treatment decisions. Elevated levels of these markers can be seen in breast cancer as well as other diseases, including ovarian cancer and colorectal cancer. Tumor marker testing is often used alongside imaging tests such as magnetic resonance imaging (MRI), CT scan, and other tests to provide a comprehensive assessment. This testing is part of routine clinical practice. It is guided by clinical practice guidelines, such as the ESMO Clinical Practice Guidelines, which outline the appropriate use of these tests in breast cancer management. It is important that test results are interpreted in the context of the patient’s current treatment, and doctors use these results, along with other clinical and laboratory findings, to make informed treatment decisions. Blood tests for tumor markers also assess blood cells and organ function, and testing may be repeated over time to monitor tumor growth or response to therapy. Breast cancer screening and diagnosis rely on a combination of imaging, pathology, and laboratory tests, and tumor markers are not used for initial breast cancer screening but may be useful in monitoring breast tumors and advanced disease. Women are most frequently affected by breast cancer, and the use of tumor markers and related tests is an important part of ongoing care for breast cancer patients.

Here, we look at some of the most common primary IHC markers for breast cancer and some cell-type-specific biomarkers. We also recommend specific IHC antibodies for each biomarker.

MUC1

MUC1 is a transmembrane glycoprotein found on the surface of epithelial cells, where it contributes to cell signaling and protection. In breast cancer, MUC1 is often overexpressed and exhibits altered glycosylation, which can promote tumor growth, invasion, and immune evasion. It is commonly used as a biomarker in diagnostic assays, including CA 15-3 and CA 27.29 blood tests. MUC1 is also being explored as a therapeutic target, with several investigational treatments in development. Its expression patterns provide insight into tumor behavior and may support personalized approaches to breast cancer management

Figure 1. Immunohistochemistry (Formalin/PFA-fixed paraffin-embedded sections) - Anti-MUC1 antibody [EPR1023] (ab109185).

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ALDH1A1

Aldehyde dehydrogenase 1A1 (ALDH1A1) is an enzyme involved in cellular detoxification and retinoic acid synthesis. In breast cancer research, ALDH1A1 has gained attention as a marker linked to cancer stem-like cells. Its expression is often observed in more aggressive subtypes, including triple-negative breast cancers. Studies suggest that ALDH1A1 may be associated with resistance to chemotherapy and poorer clinical outcomes. Researchers are exploring its potential role in identifying tumor subpopulations and predicting treatment response. While not universally expressed, ALDH1A1 remains a focus in efforts to understand tumor biology better and improve breast cancer management strategies.

Figure 2.  Immunohistochemistry (Formalin/PFA-fixed paraffin-embedded sections) - Anti-ALDH1A1 antibody [EP1933Y] - C-terminal (ab52492).

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SLFN11

Schlafen 11 (SLFN11) is a DNA/RNA-binding protein that has been studied for its role in sensitizing cancer cells to DNA-damaging agents. In breast cancer, SLFN11 expression has been linked to increased response to therapies such as topoisomerase inhibitors and platinum-based drugs. Tumors with low or absent SLFN11 may show resistance to these treatments, prompting interest in combination strategies to restore sensitivity. SLFN11 is not uniformly expressed across all breast cancer types, but its presence may help guide therapeutic decisions and patient stratification in clinical settings.

Figure 3. Immunohistochemistry (Formalin/PFA-fixed paraffin-embedded sections) - Anti-SLFN11 antibody [EPR24414-87] (ab271354).

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Estrogen receptor alpha

Estrogen receptor alpha (ERα) is a nuclear hormone receptor that regulates gene expression in response to estrogen. It is commonly expressed in hormone receptor–positive breast cancers and plays a role in tumor growth and progression. ERα status is routinely used to guide treatment decisions, particularly for endocrine therapies such as tamoxifen or aromatase inhibitors. Variants of ERα, including isoforms like ERα-LBD, have been linked to therapy resistance and altered cellular metabolism. Understanding ERα expression and function helps researchers and clinicians stratify patients and explore new approaches to managing hormone-responsive breast cancer.

Figure 4. Immunohistochemical staining of paraffin-embedded human breast carcinoma tissue with ab32063 at a dilution of 1/5000. The secondary antibody used was Goat Anti-Rabbit IgG H&L (HRP Polymer). The sample is counterstained with hematoxylin. Antigen retrieval was heat-mediated using ab93684 (Tris/EDTA buffer, pH 9.0).

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Progesterone receptor

The progesterone receptor (PR) is a nuclear hormone receptor often co-expressed with estrogen receptor alpha (ERα) in hormone-responsive breast cancers. PR is commonly used as a biomarker to assess ERα activity and guide endocrine therapy decisions. Research suggests that PR activation can influence ERα binding patterns and gene expression, potentially reducing tumor cell proliferation. PR status is typically evaluated through immunohistochemistry and may help predict treatment outcomes. While PR alone is not a standalone predictor, its presence alongside ERα supports a more nuanced understanding of hormone signaling in breast cancer biology and patient management strategies.

Figure 5. Immunohistochemistry (Formalin/PFA-fixed paraffin-embedded sections) - Anti-Progesterone Receptor antibody [YR85] (ab32085).

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ErbB2/HER2

ERBB2, also known as HER2, is a gene encoding a receptor tyrosine kinase involved in cell growth and differentiation. In about 20% of breast cancers, HER2 is overexpressed due to ERBB2 gene amplification. This overexpression promotes tumor cell proliferation through persistent activation of signaling pathways like PI3K/AKT and MAPK. HER2 often forms heterodimers with other ErbB family members, enhancing oncogenic signaling. HER2-positive breast cancers tend to show distinct biological behavior and may respond to targeted therapies. Monitoring HER2 status helps guide treatment decisions and supports ongoing research into personalized oncology strategies.

Figure 6. Immunohistochemistry (Formalin/PFA-fixed paraffin-embedded sections) - Anti-ErbB2 / HER2 antibody [EP1045Y] (ab134182)

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p53

p53 is a tumor suppressor protein encoded by the TP53 gene, known for regulating cell cycle arrest, DNA repair, and apoptosis. In breast cancer, p53 mutations are common and often associated with more aggressive tumor behavior. Wild-type p53 responds to cellular stress by activating genes limiting cell proliferation or triggering cell death. When mutated, p53 may lose this function or gain new properties that promote tumor growth. Assessing p53 status in breast tumors can provide insights into prognosis and treatment response, making it a valuable marker in breast cancer research and clinical decision-making.

Figure 7. Western blot - Anti-p53 (phospho S15) antibody [EPR64(N)] - ChIP Grade (ab223868).

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BRCA1

BRCA1 is a tumor suppressor gene involved in maintaining genomic stability through DNA repair, cell cycle control, and transcriptional regulation. Inherited mutations in BRCA1 are linked to an increased risk of breast and ovarian cancers. BRCA1 helps repair double-strand DNA breaks via homologous recombination, a precise repair mechanism. When BRCA1 function is lost, cells may accumulate genetic damage, contributing to tumor development. Testing for BRCA1 mutations can inform risk assessment and guide preventive or therapeutic strategies. Understanding BRCA1’s role continues to support research into targeted treatments and personalized approaches in breast cancer care.

Figure 8. Immunohistochemical analysis of paraffin-embedded human breast cancer tissue labeling BRCA1 with ab213929 at 1/400 dilution, followed by rabbit-specific IHC polymer detection kit HRP/DAB (ab209101).

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EGFR

EGFR, or epidermal growth factor receptor, is a transmembrane protein that belongs to the ErbB family of receptor tyrosine kinases. It plays a role in regulating cell growth, survival, and migration. In some breast cancers, particularly basal-like subtypes, EGFR is overexpressed or dysregulated, contributing to tumor progression. EGFR activation triggers downstream signaling pathways such as MAPK and PI3K/AKT. While EGFR-targeted therapies have shown limited success in breast cancer, ongoing research explores their complex signaling dynamics and potential as a biomarker. Understanding EGFR expression patterns may support more personalized approaches in breast cancer diagnosis and treatment.

Figure 9. Western blot - Anti-EGFR antibody [EP38Y] (ab52894).

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CK7 (Cytokeratin 7) and CK20 (Cytokeratin 20)

Cytokeratin 7 (CK7) and cytokeratin 20 (CK20) are intermediate filament proteins expressed in epithelial cells. Their expression patterns help pathologists distinguish between different tumor types. In breast cancer, CK7 is typically positive, while CK20 is usually absent. This CK7-positive/CK20-negative profile supports a breast origin in metastatic tumors. These markers are particularly useful when the primary site of carcinoma is unclear. Immunohistochemical staining for CK7 and CK20 contributes to diagnostic accuracy and may assist in guiding treatment strategies. Their expression profiles also help differentiate breast cancer from gastrointestinal and urothelial carcinomas, which often show different cytokeratin patterns.

Figure 10. Immunohistochemistry (Formalin/PFA-fixed paraffin-embedded sections) - Anti-Cytokeratin 7 antibody [EPR17078] - Cytoskeleton Marker (ab181598).

Figure 11. Immunohistochemistry (Formalin/PFA-fixed paraffin-embedded sections analysis of human breast carcinoma tissue labeling Cytokeratin 7 with unpurified ab68459.

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Cyclin D1

Cyclin D1 is a regulatory protein that helps control the transition from the G1 to S phase of the cell cycle. In breast cancer, overexpression of Cyclin D1 is frequently observed and is often linked to estrogen receptor (ER) positivity. The CCND1 gene, which encodes Cyclin D1, may be amplified in some tumors, contributing to increased cell proliferation. Different isoforms, such as Cyclin D1a and D1b, show distinct expression patterns and may influence prognosis. Evaluating Cyclin D1 levels can support tumor classification and may provide insight into potential therapeutic responses in hormone receptor-positive breast cancers.

Figure 12. Immunohistochemical staining of paraffin-embedded human endometrial adenocarcinoma with purified ab134175 at a dilution of 1/100. An HRP goat anti-rabbit (ab97051) was used as the secondary antibody at a dilution of 1/500, and the sample was counterstained with hematoxylin.

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Ki67

Ki67 is a nuclear protein expressed during active phases of the cell cycle, making it a widely used marker of cell proliferation. In breast cancer, Ki67 levels are assessed through immunohistochemistry to estimate tumor growth rate. Higher Ki67 expression is often associated with more aggressive tumor behavior and may influence treatment decisions, particularly in hormone receptor-positive subtypes. While interpretation can vary between laboratories, Ki67 remains a valuable tool in evaluating prognosis and guiding therapy. Ongoing studies aim to refine its clinical utility and standardize scoring methods for more consistent application in breast cancer diagnostics.

Figure 13.  Immunocytochemistry/ Immunofluorescence - Anti-Ki67 antibody [SP6] (ab16667).

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Cyclooxygenase-2 (COX-2)

Cyclooxygenase-2 (COX-2) is an inducible enzyme involved in the conversion of arachidonic acid to prostaglandins, which regulate inflammation and cell proliferation. In breast cancer, COX-2 is frequently overexpressed, particularly in aggressive subtypes such as triple-negative breast cancer. Its expression has been linked to tumor growth, angiogenesis, and resistance to apoptosis. COX-2 may also support cancer stem cell maintenance and contribute to poor prognosis. Measuring COX-2 levels can provide insight into tumor biology and may help identify patients who could benefit from COX-2-targeted therapies. Research continues to explore its role in breast cancer development and therapeutic response.

Figure 14. Immunocytochemistry/ Immunofluorescence - Anti-COX2 / Cyclooxygenase 2 antibody [EPR12012] (ab179800).

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Caveolin-1

Caveolin-1 (Cav-1) is a structural protein found in caveolae, small invaginations of the plasma membrane involved in signal transduction and lipid regulation. In breast cancer, Cav-1 expression varies between tumor cells and the surrounding stroma, influencing tumor behavior. Low stromal Cav-1 levels have been associated with more aggressive subtypes and poorer outcomes. Cav-1 also interacts with pathways like EGFR and integrins, affecting cell migration and survival. Its dual role as a tumor suppressor or promoter depends on the cancer context, making it a focus of ongoing research into breast cancer biomarkers and therapeutic strategies.

Figure 15. Immunohistochemistry (Formalin/PFA-fixed paraffin-embedded sections) - Anti-Caveolin-1 antibody [E249] - Caveolae Marker (ab32577).

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FOXA1

FOXA1 is a transcription factor that regulates chromatin accessibility and gene expression in hormone-responsive tissues. In breast cancer, FOXA1 is frequently expressed in estrogen receptor (ER)-positive and HER2-positive subtypes. It helps shape the transcriptional landscape by guiding ER binding and supporting luminal cell identity. Studies suggest that FOXA1 influences tumor progression, endocrine therapy response, and metastatic potential. Its expression patterns are being explored as biomarkers for prognosis and treatment stratification. By modulating pathways like ErbB2 and epithelial–mesenchymal transition, FOXA1 continues to be a focus in breast cancer research and biomarker development.

Figure 16. Immunohistochemistry (Formalin/PFA-fixed paraffin-embedded sections) - Anti-FOXA1 antibody [EPR10881] (ab170933).

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GATA-binding protein 3 (GATA3)

GATA-binding protein 3 (GATA3) is a transcription factor involved in the regulation of cell differentiation in various tissues, including the mammary gland. In breast cancer, GATA3 is commonly expressed in estrogen receptor-positive tumors and is used as a diagnostic marker to identify luminal subtypes. It supports luminal cell identity by regulating genes linked to epithelial characteristics. GATA3 expression has also been associated with reduced metastatic potential in triple-negative breast cancer models. Its presence in tumor profiling helps guide classification and may inform treatment strategies, making it a valuable marker in breast cancer research and diagnostics.

Figure 17. Immunohistochemistry (Formalin/PFA-fixed paraffin-embedded sections) - Anti-GATA3 antibody [EPR16651] - ChIP Grade (ab199428).

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Cytokeratin 5 (CK5)

Cytokeratin 5 (CK5) is a type of intermediate filament protein expressed in basal epithelial cells. In breast cancer, CK5 is commonly used as a marker to identify basal-like subtypes, particularly within triple-negative breast cancers. Its presence helps distinguish tumors with more aggressive behavior and limited hormone receptor expression. CK5 expression is often evaluated alongside other markers like EGFR and CK14 to refine classification. Understanding CK5 patterns can support diagnostic accuracy and inform treatment decisions. As research advances, CK5 continues to be explored for its role in tumor biology and potential as a prognostic indicator.

Figure 18. Immunohistochemistry (Formalin/PFA-fixed paraffin-embedded sections) - Anti-Cytokeratin 5 antibody [EP1601Y] - Cytoskeleton Marker (ab52635).

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Smooth muscle myosin heavy chain (SMMHC)

Smooth muscle myosin heavy chain (SMMHC) is a contractile protein typically expressed in smooth muscle cells and myoepithelial cells of the breast. In breast cancer diagnostics, SMMHC serves as a marker to identify the presence of myoepithelial cells, helping distinguish benign from malignant lesions. Its expression is often evaluated in immunohistochemistry panels alongside markers like p63 and calponin. Loss of SMMHC staining may indicate invasive carcinoma, while its presence supports a diagnosis of in situ disease. This makes SMMHC a useful tool in histopathological assessment and tumor classification in breast cancer studies.

Figure 19. Immunohistochemistry (Formalin/PFA-fixed paraffin-embedded sections) - Anti-smooth muscle Myosin heavy chain 11 antibody [EPR5336(B)] (ab133567).

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E-cadherin

E-cadherin is a calcium-dependent adhesion protein that helps maintain epithelial cell integrity by mediating cell–cell adhesion. In breast cancer, E-cadherin is widely studied for its role in distinguishing ductal from lobular carcinomas. Loss or reduction of E-cadherin expression is commonly observed in invasive lobular carcinoma, contributing to cell detachment and increased invasiveness. Its expression status is used in diagnostic panels to support tumor classification and guide treatment planning. E-cadherin also interacts with signaling pathways that influence tumor progression, making it a valuable marker in breast cancer research and histopathological evaluation.

Figure 20. Immunohistochemistry (Formalin/PFA-fixed paraffin-embedded sections) - Anti-E Cadherin antibody [EP700Y] - Intercellular Junction Marker (ab40772).

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Alpha smooth muscle actin (ACTA2)

Alpha smooth muscle actin (ACTA2) is a cytoskeletal protein expressed in smooth muscle cells and myofibroblasts. In breast tissue, ACTA2 is commonly used as a marker to identify myoepithelial cells, aiding in the distinction between benign and invasive lesions. Its expression helps pathologists confirm the presence of intact myoepithelial layers in ductal carcinoma in situ. In invasive breast cancers, ACTA2 is also found in cancer-associated fibroblasts, where it may influence tumor stiffness and progression. This dual role makes ACTA2 a valuable marker in both diagnostic pathology and tumor microenvironment research.

Figure 21. IHC image of alpha-smooth muscle actin staining in a human breast ductal carcinoma formalin-fixed paraffin-embedded tissue section, performed on a Leica Bond™ system using the standard protocol F.

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Calponin

Calponin is an actin-binding protein involved in smooth muscle contraction and cytoskeletal regulation. In breast tissue, calponin is expressed in myoepithelial cells and is commonly used as a marker in diagnostic pathology. Its presence helps distinguish benign lesions and ductal carcinoma in situ from invasive breast cancer, where myoepithelial layers are typically absent. Calponin is often used alongside markers like p63 and smooth muscle myosin heavy chain to improve diagnostic accuracy. Its role in identifying tumor boundaries and supporting histological classification makes calponin a valuable tool in breast cancer research and clinical evaluation.

Figure 22. Immunohistochemistry (Formalin/PFA-fixed paraffin-embedded sections) - Anti-Calponin 1 antibody [EP798Y] (ab46794).

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Delta-1-Catenin

Delta-1-catenin, also known as CTNND1, is a member of the catenin protein family involved in cell adhesion and signaling. It interacts with cadherins to stabilize cell–cell junctions and plays a role in maintaining epithelial integrity. In breast cancer, altered expression of delta-1-catenin has been linked to changes in tumor cell behavior, including migration and invasion. Its dysregulation may contribute to epithelial–mesenchymal transition, a process associated with metastasis. Delta-1-catenin is being studied as a potential biomarker for tumor progression and subtype classification, offering insights into breast cancer development and therapeutic response.

Figure 23. Formalin-fixed, paraffin-embedded human breast carcinoma tissue stained for delta 1 Catenin/CAS using ab227638 at 1/100 dilution in immunohistochemical analysis.

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Cytokeratin 14 (CK14)

Cytokeratin 14 (CK14) is a type I intermediate filament protein typically expressed in basal epithelial cells. In breast cancer, CK14 is used as a biomarker to identify basal-like subtypes, particularly within triple-negative breast cancers. Its expression is associated with a more undifferentiated phenotype and may indicate a more aggressive tumor behavior. CK14 is often evaluated alongside markers like CK5 and EGFR to refine molecular classification. Understanding CK14 expression patterns supports diagnostic accuracy and helps guide research into tumor origin, progression, and potential therapeutic targets in breast cancer.

Figure 24.  Immunohistochemical analysis of paraffin-embedded human squamous cell carcinoma of cervix tissue labeling Cytokeratin 14 with ab181595 at 1/2000 dilution, followed by prediluted HRP polymer for Rabbit/Mouse IgG.

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