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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.
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+.
In plain words · A growth-signal receptor. Some cancers make far too much of it, and drugs that block it or use it as a docking site have transformed those cancers.
A 1+ result still counts as HER2-negative for trastuzumab, but in breast cancer it now means HER2-low, and trastuzumab deruxtecan is on label once the cancer has spread and earlier chemotherapy or hormone therapy has stopped working. If your report is older than 2022 the 1+ score may not have been mentioned to you; it is worth asking.
Written only from the label or guideline text cited on this page. Not medical advice; your own report and the reading your team gives it come first.
1+: incomplete membrane staining that is faint or barely perceptible in more than 10 percent of tumour cells; HER2-negative under ASCO/CAP, HER2-low for trastuzumab deruxtecan.
“HER2-low (IHC 1+ or IHC 2+/ISH-) breast cancer, as determined by an FDA-authorized test”
ENHERTU prescribing information; scoring band from the ASCO/CAP 2018 guideline| Threshold | Drug | Cancer | Regulator | Source |
|---|---|---|---|---|
| IHC 1+ (HER2-low) | Trastuzumab deruxtecan | HER2-low and HER2-ultralow metastatic breast cancer | FDA | label |
| Device | Maker | Indication and sample | Drug | PMA / 510(k) |
|---|---|---|---|---|
| PATHWAY anti-Her2/neu (4B5) Rabbit Monoclonal Primary Antibody | Ventana Medical Systems (Roche) | Breast Cancer - Tissue | Trastuzumab deruxtecan | P990081/S047 (09/30/2022) |
Matched on the name and aliases of the readout in the title, setting and summary of each trial; a match is a mention, not proof the readout was an entry criterion.
Shares HER2 IHC 0 (HER2-negative, including ultralow), HER2-low (IHC 1+ or IHC 2+/ISH-negative), HER2 IHC and ISH companion assays (HercepTest, PATHWAY 4B5, HER2 Dual ISH), HER2 IHC 3+ (HER2-positive by immunohistochemistry) and the tags biomarker, her2.
Shares HER2 IHC 2+ (equivocal, reflex to ISH), HER2 IHC and ISH companion assays (HercepTest, PATHWAY 4B5, HER2 Dual ISH), HER2 IHC 3+ (HER2-positive by immunohistochemistry), Trastuzumab deruxtecan and the tags biomarker, her2.
Shares HER2 IHC 3+ (HER2-positive by immunohistochemistry), Trastuzumab deruxtecan, HER2 and the tags biomarker, her2.
Shares HER2 IHC 0 (HER2-negative, including ultralow), HER2-low (IHC 1+ or IHC 2+/ISH-negative), HER2 IHC and ISH companion assays (HercepTest, PATHWAY 4B5, HER2 Dual ISH), HER2-low and HER2-ultralow.
Shares HER2-low (IHC 1+ or IHC 2+/ISH-negative), HER2 IHC and ISH companion assays (HercepTest, PATHWAY 4B5, HER2 Dual ISH), HER2-low and HER2-ultralow metastatic breast cancer, HER2-low and HER2-ultralow.
Shares HER2 IHC 3+ (HER2-positive by immunohistochemistry), Immunohistochemistry (IHC) and the tag biomarker.
Shares Immunohistochemistry (IHC), Breast cancer (all types), HR-positive / HER2-negative breast cancer and the tag biomarker.
Shares HER2 IHC 3+ (HER2-positive by immunohistochemistry), Breast cancer (all types) and the tag biomarker.
Every source this page cites, grouped by who published it. Grouping is read from the web address, not assigned by hand.
A source that has moved or gone is reported by the weekly link check. /audit/