HER3 Results Explained

expression, ERBB3 mutations and antibody-drug conjugate relevance

What each line of the report means

Specimen: Tumor tissue or blood, as stated
HER3 protein expression is measured on tissue; ERBB3 mutations can appear on tissue or blood sequencing panels.
Biomarker / finding: HER3 expression by IHC, or an ERBB3 mutation
Protein expression and gene mutation are two different findings and are not interpreted the same way.
Quantitative result: IHC intensity and percentage, or variant allele frequency
There is no single universally agreed HER3 positivity cutoff in routine practice.
Clinical interpretation: Trial and context review required
HER3 is mainly relevant through antibody-drug conjugates and resistance biology rather than a standard treatment switch.

How this result is interpreted

  • HER3 is widely expressed across breast, lung, colorectal and other cancers, so the presence of expression alone is less informative than the degree and the clinical context.
  • HER3 activity is a recognized escape route from EGFR-directed and endocrine therapy, which is one reason it appears in reports after progression.
  • HER3-directed antibody-drug conjugates are an active area of development, so the practical value of the result is often trial access rather than an approved treatment today.

What this result cannot tell you

  • Unlike HER2, HER3 has no long-established scoring standard used consistently across laboratories, so results are harder to compare.
  • An ERBB3 mutation on a panel is not automatically a driver; many are of uncertain significance and need variant-level review.
  • Expression measured before several lines of therapy may not represent current biology.

Questions to ask your oncologist

  • Is this a protein expression result, a gene mutation, or both?
  • What assay and cutoff did the laboratory use, and how was positivity defined?
  • Is this finding being used to explain resistance to a previous treatment, or to open a new option?
  • Are there HER3-directed trials relevant to my cancer type and line of therapy?
  • Should this be reviewed alongside my HER2, EGFR or hormone receptor results?

Frequently asked questions

Is HER3 the same as HER2?

No. They are related receptors in the same family and can pair with one another, but they are tested differently, scored differently and lead to different treatments. A HER3 result does not change HER2 status.

Is there an approved HER3-targeted drug for my cancer?

Availability changes as agents move through review. In many situations HER3 findings currently point toward clinical trials, so the useful question is which trials are open for your cancer type and line of therapy.

Can someone independently review this result?

Yes. A Clarity Report can assess whether the finding is expression or mutation, how it fits your treatment history, and which trials are relevant, then return the findings in writing.

Sources

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