Abstract
Accurate identification of HER2-low breast cancers remains challenging using standard IHC. We analyzed 3182 breast tumors using transcriptomics, revealing detectable ERBB2 mRNA in 86% of IHC 0 cases. Pathological complete response was most prevalent in anti-HER2-treated patients with highest ERBB2 expression. Our results demonstrate that transcriptomics can sensitively detect HER2 expression and stratify patients beyond IHC classification, supporting its use as a complementary biomarker for guiding anti-HER2 therapy decisions.