IHC / ISH testing; spans HER2-positive, HER2-low, and HER2-ultralow categories

HER2 / ERBB2

  • Human epidermal growth factor receptor 2 (HER2), encoded by ERBB2; a receptor tyrosine kinase of the HER/ErbB family
  • Gene Location: Chromosome 17 (17q12)

Biology

  • HER2 is a transmembrane receptor tyrosine kinase with no known ligand. It heterodimerizes with other HER-family receptors, particularly HER3, to activate PI3K/AKT and MAPK signaling that drives proliferation and survival.1
  • In HER2-positive tumors, ERBB2 gene amplification and protein overexpression drive aggressive tumor growth. HER2 status is both prognostic and predictive of benefit from HER2-directed therapy.1
  • Three drug classes target the receptor: monoclonal antibodies that block dimerization and recruit immune effector function, antibody-drug conjugates that use HER2 as a delivery target for a cytotoxic payload, and small-molecule tyrosine kinase inhibitors that block intracellular signaling. Only the antibody-drug conjugates retain activity when HER2 expression is low, because payload delivery and bystander killing do not require pathway dependence.4,6

Etiology and Epidemiology

  • HER2 is amplified or overexpressed in approximately 15% to 20% of breast cancers (HER2-positive), historically associated with more aggressive disease and, before HER2-targeted therapy, poorer outcomes.2
  • HER2-low (IHC 1+, or IHC 2+ with negative in situ hybridization) is a therapeutically actionable expression range rather than a distinct biologic subtype. Approximately 60% to 65% of HR-positive/HER2-negative breast cancers fall into this category.3
  • HER2-ultralow (IHC 0 with faint, incomplete membrane staining in 10% or fewer tumor cells) was operationally defined in the DESTINY-Breast06 trial. Approximately 20% to 25% of HR-positive/HER2-negative tumors are HER2-ultralow.3,4
  • The practical consequence is that the historical binary of HER2-positive versus HER2-negative no longer maps to treatment eligibility. A tumor reported as HER2-negative may still be HER2-low or HER2-ultralow and therefore eligible for trastuzumab deruxtecan.4

Testing

When to Test:

  • HER2 status should be determined on all newly diagnosed invasive breast cancers and, when feasible, on recurrences or metastases, since status can change between primary and metastatic disease.1
  • Because HER2-low and HER2-ultralow now carry treatment implications, a prior report of HER2-negative is not sufficient to exclude a patient from HER2-directed therapy. Re-review of existing slides may be required to distinguish IHC 0 with faint staining from true IHC 0.4

Testing Methodologies:

  • Immunohistochemistry is the standard initial assay, scored 0, 1+, 2+, or 3+. Equivocal IHC 2+ results are reflexed to in situ hybridization.1
  • HER2-positive is defined as IHC 3+ or IHC 2+ with amplified in situ hybridization. HER2-negative is IHC 0, 1+, or 2+ without amplification. The Ventana PATHWAY anti-HER-2/neu (4B5) assay was used to determine eligibility in the DESTINY-Breast trials.1,4
  • Distinguishing IHC 0 from IHC 1+, and identifying HER2-ultralow within IHC 0, requires careful scoring at the low end of the dynamic range where the assay was never designed to discriminate. Interobserver concordance is lower in this range than for HER2-positive determination.1,3

Guideline Recommendations

  • The ASCO/CAP HER2 Testing in Breast Cancer guideline defines immunohistochemistry and in situ hybridization scoring. Its 2023 update did not endorse HER2-low as a formal reporting category but addressed reporting of low-level expression.1
  • NCCN incorporates HER2 immunohistochemistry and in situ hybridization status, including HER2-low and HER2-ultralow expression, to determine eligibility for trastuzumab deruxtecan and other HER2-directed therapy.5

Targeted Therapy

Approved Agents

  • Eight HER2-directed agents are FDA-approved in breast cancer across three classes: antibody-drug conjugates (trastuzumab deruxtecan, ado-trastuzumab emtansine), monoclonal antibodies (trastuzumab, pertuzumab, margetuximab), and tyrosine kinase inhibitors (tucatinib, neratinib, lapatinib).
  • Trastuzumab deruxtecan is the only agent indicated beyond HER2-positive disease, carrying HER2-low and HR-positive HER2-ultralow metastatic indications in addition to HER2-positive early and metastatic settings.4
  • In 2025, the FDA approved trastuzumab deruxtecan plus pertuzumab for the first-line treatment of unresectable or metastatic HER2-positive breast cancer based on DESTINY-Breast09, the first new first-line option in more than a decade. Median progression-free survival was 40.7 versus 26.9 months versus a taxane, trastuzumab, and pertuzumab (HR 0.56; 95% CI 0.44-0.71; P<0.0001). In 2026, neoadjuvant and adjuvant early breast cancer indications were added based on DESTINY-Breast11 and DESTINY-Breast05.4,9

Antibody-Drug Conjugates

Trastuzumab Deruxtecan (Enhertu)

  • FDA-Approved Indication:
    • Spans HER2-positive early and metastatic disease, including neoadjuvant and adjuvant treatment and first-line metastatic treatment in combination with pertuzumab.4
    • Also indicated for unresectable or metastatic HER2-low (IHC 1+ or IHC 2+/ISH-negative) and HR-positive HER2-ultralow breast cancer after prior therapy.4
  • Mechanism of Action:
    • HER2-directed antibody-drug conjugate joining trastuzumab to the topoisomerase I inhibitor DXd through a cleavable tetrapeptide linker.4
    • A high drug-to-antibody ratio and membrane-permeable payload produce a bystander effect, enabling activity across a wide range of HER2 expression. Carries a boxed warning for interstitial lung disease and embryo-fetal toxicity.4

Drug Information:

Learn more about Trastuzumab Deruxtecan ›

Patient Resources:

https://www.enhertu.com/

Ado-Trastuzumab Emtansine (Kadcyla)

  • FDA-Approved Indication:
    • HER2-positive metastatic breast cancer in patients who previously received trastuzumab and a taxane, separately or in combination (EMILIA).6
    • Adjuvant treatment of HER2-positive early breast cancer with residual invasive disease after neoadjuvant taxane and trastuzumab-based treatment (KATHERINE).6
  • Mechanism of Action:
    • HER2-directed antibody-drug conjugate joining trastuzumab to the microtubule inhibitor DM1 through a non-cleavable linker.6
    • Carries a boxed warning for hepatotoxicity, cardiac toxicity, and embryo-fetal toxicity.6

Drug Information:

Learn more about Ado-Trastuzumab Emtansine ›

Patient Resources:

https://www.kadcyla.com/

HER2-Directed Monoclonal Antibodies

Trastuzumab (Herceptin)

  • FDA-Approved Indication:
    • Adjuvant treatment of HER2-overexpressing node-positive or node-negative breast cancer, as part of a regimen with chemotherapy or as a single agent following multi-modality anthracycline-based therapy.7
    • First-line treatment of HER2-overexpressing metastatic breast cancer in combination with paclitaxel, and as a single agent after one or more chemotherapy regimens for metastatic disease.7
  • Mechanism of Action:
    • Humanized monoclonal antibody binding subdomain IV of the HER2 extracellular domain, inhibiting ligand-independent signaling and mediating antibody-dependent cell-mediated cytotoxicity.7
    • The foundational HER2-directed agent, and the backbone on which most modern combinations are built. Carries a boxed warning for cardiomyopathy, infusion reactions, embryo-fetal toxicity, and pulmonary toxicity.7

Drug Information:

Learn more about Trastuzumab ›

Patient Resources:

https://www.herceptin.com/

Pertuzumab (Perjeta)

  • FDA-Approved Indication:
    • In combination with trastuzumab and docetaxel for first-line treatment of HER2-positive metastatic breast cancer (CLEOPATRA).8
    • In combination with trastuzumab and chemotherapy for the neoadjuvant treatment of HER2-positive locally advanced, inflammatory, or early-stage breast cancer, and for adjuvant treatment of HER2-positive early breast cancer at high risk of recurrence (APHINITY).8
  • Mechanism of Action:
    • Humanized monoclonal antibody binding subdomain II of HER2, blocking ligand-dependent HER2/HER3 heterodimerization.8
    • Because it binds a different epitope than trastuzumab, the two provide complementary rather than redundant blockade. Carries a boxed warning for cardiomyopathy and embryo-fetal toxicity.8

Drug Information:

Learn more about Pertuzumab ›

Patient Resources:

https://www.perjeta.com/

Margetuximab (Margenza)

  • FDA-Approved Indication:
    • In combination with chemotherapy for metastatic HER2-positive breast cancer in adults who have received two or more prior anti-HER2 regimens, at least one for metastatic disease (SOPHIA).10
  • Mechanism of Action:
    • Fc-engineered anti-HER2 monoclonal antibody. The modified Fc region increases affinity for the activating Fc-gamma receptor CD16A and decreases affinity for the inhibitory receptor CD32B, enhancing antibody-dependent cell-mediated cytotoxicity.10
    • Carries a boxed warning for left ventricular dysfunction and embryo-fetal toxicity.10

Drug Information:

Learn more about Margetuximab ›

Patient Resources:

https://www.margenza.com/

HER2 Tyrosine Kinase Inhibitors

Tucatinib (Tukysa)

  • FDA-Approved Indication:
    • In combination with trastuzumab and capecitabine for advanced unresectable or metastatic HER2-positive breast cancer, including patients with brain metastases, who have received one or more prior anti-HER2-based regimens in the metastatic setting (HER2CLIMB).11
  • Mechanism of Action:
    • Highly HER2-selective small-molecule tyrosine kinase inhibitor with minimal EGFR inhibition, which accounts for a lower rate of EGFR-type toxicity such as rash and diarrhea than less selective agents.11
    • The only HER2-directed agent whose registrational trial specifically enrolled and demonstrated intracranial activity in patients with active brain metastases.11

Drug Information:

Learn more about Tucatinib ›

Patient Resources:

https://www.tukysa.com/

Neratinib (Nerlynx)

  • FDA-Approved Indication:
    • Extended adjuvant treatment of adults with early-stage HER2-positive breast cancer following adjuvant trastuzumab-based therapy (ExteNET).12
    • In combination with capecitabine for adults with advanced or metastatic HER2-positive breast cancer who have received two or more prior anti-HER2-based regimens in the metastatic setting (NALA).12
  • Mechanism of Action:
    • Irreversible pan-HER tyrosine kinase inhibitor targeting HER1 (EGFR), HER2, and HER4.12
    • Diarrhea is the dominant toxicity and antidiarrheal prophylaxis is required from the first dose.12

Drug Information:

Learn more about Neratinib ›

Patient Resources:

https://www.nerlynx.com/

Lapatinib (Tykerb)

  • FDA-Approved Indication:
    • In combination with capecitabine for advanced or metastatic HER2-positive breast cancer in patients who have received prior therapy including an anthracycline, a taxane, and trastuzumab.13
    • In combination with letrozole for postmenopausal women with hormone receptor-positive, HER2-positive metastatic breast cancer for whom hormonal therapy is indicated.13
  • Mechanism of Action:
    • Reversible dual tyrosine kinase inhibitor of HER2 and EGFR (HER1).13
    • Carries a boxed warning for hepatotoxicity.13

Drug Information:

Learn more about Lapatinib ›

Patient Resources:

https://www.novartis.com/us-en/medicines

References

  1. Wolff AC, Somerfield MR, Dowsett M, et al. Human epidermal growth factor receptor 2 testing in breast cancer: ASCO-College of American Pathologists guideline update. J Clin Oncol. 2023;41(22):3867-3872. doi:10.1200/JCO.22.02864
  2. Howlader N, Altekruse SF, Li CI, et al. US incidence of breast cancer subtypes defined by joint hormone receptor and HER2 status. J Natl Cancer Inst. 2014;106(5):dju055. doi:10.1093/jnci/dju055
  3. Bardia A, Hu X, Dent R, et al. Trastuzumab deruxtecan after endocrine therapy in metastatic breast cancer. N Engl J Med. 2024;391(22):2110-2122.
  4. ENHERTU (fam-trastuzumab deruxtecan-nxki) for injection. Prescribing information. Daiichi Sankyo, Inc. Accessed August 2026.
  5. NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines®) for Breast Cancer V.5.2026. © National Comprehensive Cancer Network, Inc. 2026. All rights reserved. Accessed August 2026. To view the most recent and complete version of the guideline, go online to NCCN.org.
  6. KADCYLA (ado-trastuzumab emtansine) for injection. Prescribing information. Genentech, Inc. Accessed August 2026.
  7. HERCEPTIN (trastuzumab) for injection. Prescribing information. Genentech, Inc. Accessed August 2026.
  8. PERJETA (pertuzumab) injection. Prescribing information. Genentech, Inc. Accessed August 2026.
  9. AstraZeneca and Daiichi Sankyo. ENHERTU plus pertuzumab approved in the US as first new treatment in more than a decade for the first-line treatment of patients with HER2-positive metastatic breast cancer. News release. December 15, 2025. Accessed August 2026.
  10. MARGENZA (margetuximab-cmkb) injection. Prescribing information. Accessed August 2026.
  11. 11. TUKYSA (tucatinib) tablets. Prescribing information. Seagen Inc. Accessed August 2026.
  12. 12. NERLYNX (neratinib) tablets. Prescribing information. Puma Biotechnology, Inc. Accessed August 2026.
  13. 13. TYKERB (lapatinib) tablets. Prescribing information. Novartis Pharmaceuticals Corporation. Accessed August 2026.