Biomarker Consortium

Breast Cancer

HER2

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

Human epidermal growth factor receptor 2 (HER2), encoded by 

; a receptor tyrosine kinase of the HER/ErbB family

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.

 gene amplification and protein overexpression drive aggressive tumor growth. HER2 status is both prognostic and predictive of benefit from HER2-directed therapy.

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.

HER2 is amplified or overexpressed in approximately 

 of breast cancers (HER2-positive), historically associated with more aggressive disease and, before HER2-targeted therapy, poorer outcomes.

 (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.

 (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.

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.

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.

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.

Immunohistochemistry is the standard initial assay, scored 0, 1+, 2+, or 3+. Equivocal IHC 2+ results are reflexed to in situ hybridization.

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.

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.

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.

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.

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.

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.

Spans HER2-positive early and metastatic disease, including neoadjuvant and adjuvant treatment and first-line metastatic treatment in combination with pertuzumab.

Also indicated for unresectable or metastatic 

 (IHC 1+ or IHC 2+/ISH-negative) and HR-positive 

HER2-directed antibody-drug conjugate joining trastuzumab to the topoisomerase I inhibitor DXd through a cleavable tetrapeptide linker.

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.

Learn more about Trastuzumab Deruxtecan ›

HER2-positive metastatic breast cancer in patients who previously received trastuzumab and a taxane, separately or in combination (EMILIA).

Adjuvant treatment of HER2-positive early breast cancer with residual invasive disease after neoadjuvant taxane and trastuzumab-based treatment (KATHERINE).

HER2-directed antibody-drug conjugate joining trastuzumab to the microtubule inhibitor DM1 through a non-cleavable linker.

Carries a boxed warning for hepatotoxicity, cardiac toxicity, and embryo-fetal toxicity.

Learn more about Ado-Trastuzumab Emtansine ›

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.

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.

Humanized monoclonal antibody binding subdomain IV of the HER2 extracellular domain, inhibiting ligand-independent signaling and mediating antibody-dependent cell-mediated cytotoxicity.

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.

In combination with trastuzumab and docetaxel for first-line treatment of HER2-positive metastatic breast cancer (CLEOPATRA).

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).

Humanized monoclonal antibody binding subdomain II of HER2, blocking ligand-dependent HER2/HER3 heterodimerization.

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.

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).

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.

Carries a boxed warning for left ventricular dysfunction and embryo-fetal toxicity.

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).

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.

The only HER2-directed agent whose registrational trial specifically enrolled and demonstrated intracranial activity in patients with active brain metastases.

Biomarkers

In combination with trastuzumab and capecitabine for the treatment of adults with advanced unresectable or metastatic HER2-positive breast cancer, 

, who have received one or more prior anti-HER2-based regimens in the metastatic setting.

300 mg orally twice daily, with or without food, in combination with trastuzumab and capecitabine, until disease progression or unacceptable toxicity. Swallow tablets whole; do not chew, crush, or split.

Capecitabine is given at 1,000 mg/m2 orally twice daily on Days 1 to 14 of each 21-day cycle. Trastuzumab is given per its standard schedule.

Assess liver function tests before starting, every 3 weeks during treatment, and as clinically indicated.

Reduce the dose to 150 mg twice daily in patients with severe hepatic impairment (Child-Pugh C). Avoid concomitant strong CYP3A or moderate CYP2C8 inducers and strong CYP2C8 inhibitors.

HER2-positive metastatic breast cancer previously treated with trastuzumab, pertuzumab, and ado-trastuzumab emtansine. Notably, the trial enrolled patients with 

previously treated stable brain metastases, previously treated progressing brain metastases, and untreated brain metastases

; 612 patients randomized, of whom approximately 48% had brain metastases at baseline.

Tucatinib 300 mg twice daily plus trastuzumab and capecitabine versus placebo plus trastuzumab and capecitabine, randomized 2:1.

HER2CLIMB is the registrational trial that specifically enrolled patients with active and untreated brain metastases and demonstrated intracranial benefit, which is why the indication explicitly names patients with brain metastases. This distinguishes tucatinib from other HER2-directed agents whose CNS data are retrospective or subgroup-derived.

diarrhea, palmar-plantar erythrodysesthesia, nausea, fatigue, hepatotoxicity, vomiting, stomatitis, decreased appetite, abdominal pain, headache, anemia, and rash.

diarrhea is the most frequent adverse reaction and can be severe, with cases of dehydration, hypotension, acute kidney injury, and death reported. Antidiarrheal treatment should be started at onset as clinically indicated; prophylaxis is not required, in contrast to neratinib.

elevations in ALT, AST, and bilirubin occurred. Monitor liver function tests before starting, every 3 weeks during treatment, and as clinically indicated, with dose modification for Grade 3 or higher elevations.

Because tucatinib is highly HER2-selective with minimal EGFR inhibition, rates of EGFR-type toxicity such as rash are lower than with the less selective pan-HER inhibitors.

TUKYSA (tucatinib) tablets. Prescribing information. Seagen Inc. Accessed August 2026.

Murthy RK, Loi S, Okines A, et al. Tucatinib, trastuzumab, and capecitabine for HER2-positive metastatic breast cancer (HER2CLIMB). 