Abstract Background: HER2 expression or amplification is now an established companion diagnostic test. Now, as HER2-targeting neoadjuvant therapy for breast cancer increases, the accuracy of the companion diagnostic tests are revisited. Specifically, we seek to determine with the level of expression of the target (HER2 protein) is associated with pathologic response. In this study, we quantitatively assess HER2 expression both before and after the neoadjuvant therapies to breast cancer in the BrUOG BR-211B (NCT00617942) trial. Methods: We evaluated core biopsy samples from breast cancer patients enrolled in a preoperative clinical trial using trastuzumab, nab-paclitaxel and carboplatin combination therapy. Tumor core biopsies were taken before and after 10 days of either trastuzumab or nab-paclitaxel treatment. Combination chemotherapy was given for 6 cycles. Automated quantitative analysis (AQUA), a fluorescent-based method for analysis of in situ protein expression, was used to assess HER2 and phospho-HER2 expression in the BruOG Cohort, a study where patients who serially received Herceptin and Abraxane were biopsied both before and after treatment. This study assesses the HER2 positive patient subset (n=27) Results: Of 18 evaluable patients, the 8 with a PathCR showed a mean HER2 levels of 10251 compared with 4766 in the patients without PathCR (p=0.0047). This is notable in that a HER2 score of 2000 is equivalent to HER2 2+ in previous studies. Measurement of Phospho-HER2 showed no difference in PathCR vs Non-PathCR groups. A small subset (9) cases were measured for HER2 immediately after treatment, but the study has accrued an insufficient number of cases to statistically determine if there is a reduction in HER2 immediately following treatment. Conclusions: High levels of HER2 are associated with PathCR in the neoadjuvant setting. Levels of Phospho-HER2 are not associated with response. Further studies are underway to determine changes in levels of HER2 immediately after treatment. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 705. doi:1538-7445.AM2012-705