# HER2-low (IHC 1+ or IHC 2+/ISH-negative)

Source: https://onco.cc/biomarkers/her2-low-ihc/  
OnCo record `her2-low-ihc` (Biomarker). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

HER2-low is not a new stain but a new reading of the old one: 1+ or 2+ without gene amplification. It covers about half of breast cancers and makes them eligible for trastuzumab deruxtecan.

## Summary

HER2-low is defined in the trastuzumab deruxtecan label as IHC 1+ or IHC 2+ with a negative in situ hybridisation result, determined by an FDA-authorised test. DESTINY-Breast04 (2022) established the category in metastatic breast cancer after one or two lines of chemotherapy, and DESTINY-Breast06 (2025) extended it, with HER2-ultralow, to HR-positive disease after endocrine therapy. The FDA companion diagnostic list carries the PATHWAY 4B5 antibody with the detail 'HER2-low expression (IHC 1+ or IHC 2+/ISH non-amplified)'. Outside breast cancer HER2-low is a trial category, not a label one.

## Fields

- Kind: Biomarker
- Last checked: 2026-09-23
- Also known as: HER2-low breast cancer; HER2-low readout; IHC 1+ or 2+/ISH-; HER2 low expression; HER2-low (IHC 1+ or IHC 2+/ISH-)
- Tags: biomarker; her2

## Sources

- ENHERTU prescribing information (DailyMed): https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=7e67e73e-ddf4-4e4d-8b50-09d7514910b6

## Connected records

- biomarkers: [HER2 IHC 0 (HER2-negative, including ultralow)](https://onco.cc/biomarkers/her2-ihc-0/), [HER2 IHC 1+](https://onco.cc/biomarkers/her2-ihc-1-plus/), [HER2 IHC 2+ (equivocal, reflex to ISH)](https://onco.cc/biomarkers/her2-ihc-2-plus/), [HER2 IHC 3+ (HER2-positive by immunohistochemistry)](https://onco.cc/biomarkers/her2-ihc-3-plus/), [HER2-ultralow (IHC 0 with membrane staining)](https://onco.cc/biomarkers/her2-ultralow/)
- cancers: [Breast cancer (all types)](https://onco.cc/cancers/breast-cancer/), [HER2-low and HER2-ultralow metastatic breast cancer](https://onco.cc/cancers/her2-low-metastatic-breast-cancer/), [HR-positive / HER2-negative breast cancer](https://onco.cc/cancers/breast-hr-positive/), [Metastatic triple-negative breast cancer](https://onco.cc/cancers/tnbc-metastatic/)
- drugs: [HER2 IHC and ISH companion assays (HercepTest, PATHWAY 4B5, HER2 Dual ISH)](https://onco.cc/drugs/her2-testing-assays/), [Trastuzumab deruxtecan](https://onco.cc/drugs/trastuzumab-deruxtecan/)
- terms: [FISH / ISH (in situ hybridisation)](https://onco.cc/terms/fish/), [HER2-low and HER2-ultralow](https://onco.cc/terms/her2-low/), [Immunohistochemistry (IHC)](https://onco.cc/terms/ihc/)
- targets: [HER2](https://onco.cc/targets/her2/)

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