Background: Breast cancer patients with HER2 low expression by immunohistochemistry (IHC), defined as IHC1+ or IHC2+ without gene amplification (ISH-) do not respond to conventional anti-HER2 therapies such as trastuzumab or pertuzumab. The HER2-targeted antibody-drug conjugate (ADC) trastuzumab deruxtecan (T-DXd) showed efficacy in late line HER2-overexpressing patients, with some responses in patients with low HER2 expression. Two of the market leading IHC IVDs are Dako Herceptest and Ventana 4B5. T-DXd is being investigated in HER2 low patients as determined by the VENTANA anti-HER2/neu (4B5) assay in the phase III DESTINY-Breast04 study. There is a paucity of information on prevalence of IHC1+/2+ in different breast cancer subtypes, which this study aims to address. Methods: HER2 status was calculated from 3750 consecutive primary or metastatic breast cancer patient samples successfully stained using the 4B5 assay and scored locally according to ASCO/CAP 2018 guidelines. Samples were obtained from 3 anatomic pathology labs that support networks of US community hospitals. 500 additional breast cancer samples, pre-selected to include a range of IHC staining (concordance cohort), were stained with both 4B5 and HercepTest (Dako/Agilent) and scored (IHC0, IHC1+, IHC2+, IHC3+) at a central laboratory. Results: Prevalence of HER2 categories in 3750 consecutive breast cancer samples is presented in the table below. >50% of estrogen-receptor positive (ER+ve) and progesterone receptor positive (PR+ve) subtypes are HER2 low. In the 500 sample concordance cohort, 28.0% were IHC1+/2+ using the 4B5 assay compared with 11.6% using HercepTest. HercepTest identified IHC1+/2+ staining in 21.6% [95%CI:15.1,29.4] of the patients classified as IHC1+/2+ by 4B5. 98.3% [95%CI: 96.2, 99.5] of samples IHC0 by 4B5 were also IHC0 by Herceptest. Conclusions: HER2 IHC1+/2+ (ISH-) by Ventana 4B5 represent a significant proportion of breast cancer patients and more than 50% of ER+ve and PR+ve subtypes. The 4B5 assay classed several patients as IHC1+/2+ that are IHC0 by Herceptest, but almost all patients IHC0 by 4B5 were also IHC0 by Herceptest.