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. 2026 Jul 8;28:166. doi: 10.1186/s13058-026-02334-4

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© The Author(s) 2026

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 2 — ERBB2 mRNA expression levels according to HER2 status in the overall cohort and stratified by hormone receptor subtype. Boxplots illustrate ERBB2 mRNA expression (2^− ΔCT, relative to RPL0) in HER2-zero, HER2-low, HER2-null, and HER2-ultralow tumors. For HER2-zero versus HER2-low comparison, boxplot A illustrates the overall HER2-negative population (n = 173), B hormone receptor-positive subgroup (n = 106), and C triple-negative breast cancer subgroup (n = 64). For HER2-null versus HER2-ultralow comparison, boxplot D illustrates the overall HER2-zero population, E hormone receptor-positive tumors, and F triple-negative breast cancer (n = 33). ERBB2 expression was significantly higher in HER2-ultralow tumors in the overall cohort (p = 0.009), with no statistically significant differences observed in the hormone receptor-positive (p = 0.20) or triple-negative breast cancer subgroups (p = 0.09) (Mann-Whitney U test). Horizontal lines indicate median values; boxes represent interquartile ranges; whiskers indicate range