# HER2 IHC 1+

Source: https://onco.cc/biomarkers/her2-ihc-1-plus/  
OnCo record `her2-ihc-1-plus` (Biomarker). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

IHC 1+ is faint, incomplete HER2 staining. It was called HER2-negative for twenty years; since 2022 it is the larger half of HER2-low, which trastuzumab deruxtecan treats in breast cancer.

## Summary

1+ is incomplete, faint or barely perceptible membrane staining in more than 10 percent of tumour cells. Under the ASCO/CAP guideline it is HER2-negative for trastuzumab purposes, and no gene test is needed. The DESTINY-Breast04 trial (2022) and DESTINY-Breast06 (2025) defined HER2-low as IHC 1+ or IHC 2+/ISH-, and trastuzumab deruxtecan's US label adopts that wording, so a 1+ result now selects for an antibody-drug conjugate that the parent antibody trastuzumab would not be given for. About half of breast cancers once labelled HER2-negative fall into 1+.

## Fields

- Kind: Biomarker
- Last checked: 2026-09-23
- Also known as: HER2 IHC 1+; HER2 1+; IHC 1+; HER2 1+ breast cancer
- Tags: biomarker; her2

## Sources

- ENHERTU prescribing information (DailyMed): https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=7e67e73e-ddf4-4e4d-8b50-09d7514910b6
- ASCO/CAP HER2 testing in breast cancer guideline, 2018 focused update (Wolff et al., J Clin Oncol): https://doi.org/10.1200/JCO.2018.77.8738

## Connected records

- biomarkers: [HER2 IHC 0 (HER2-negative, including ultralow)](https://onco.cc/biomarkers/her2-ihc-0/), [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-low (IHC 1+ or IHC 2+/ISH-negative)](https://onco.cc/biomarkers/her2-low-ihc/)
- 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/)
- 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: [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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