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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.