HER2 IHC 1+

aka HER2 IHC 1+, HER2 1+, IHC 1+, HER2 1+ breast cancer

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+.

Part ofHER2· ERBB2Other readoutsHER2 HER2 IHC 0 HER2 IHC 2+HER2 IHC 3+HER2 ISH amplified HER2-low HER2-ultralow All readouts

HER2: what it is and how drugs act on it · animated schematic, not to scale

Target · the protein and the cell it sits on
Drug · antibody, small molecule, cell or radioligand
Effect · signal, damage or kill

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.

IHC score 1 approval threshold

What it means for a patient

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.

Scoring rule

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

Thresholds used in approvals

Threshold	Drug	Cancer	Regulator	Source
IHC 1+ (HER2-low)	Trastuzumab deruxtecan	HER2-low and HER2-ultralow metastatic breast cancer	FDA	label

Drugs whose label names it

Trastuzumab deruxtecan

Companion diagnostics (FDA list)

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)

Tests that measure it

HercepTest PATHWAY anti-HER2/neu (4B5)

Trials that name this readout

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.