11014 Background: ER and HER2 measurements underpin a majority of breast cancer clinical testing, yet determination of ER and HER2 protein levels are routinely performed using subjective approaches. We have developed a semi-quantitative calibrated protein microarray, the Reverse Phase Protein Microarray (RPMA), to more adequately determine accurate and precise protein expression levels in clinical tissue specimens. We utilized this method to determine HER2 and ER expression levels and compared the results to those values reported from the clinical laboratory. Methods: Pure tumor epithelium from 149 frozen pre-treatment human breast cancer tissue specimens (from the I-SPY TRIAL: CALGB 150007/150012, ACRIN 6657) were procured via Laser Capture Microdissection and protein pathway mapping was performed whereby the ER (N=112) and HER2 (N= 118) levels were directly measured and compared with reported IHC values. Results: Overall, RPMA measurements of HER2 had excellent correlation with IHC and FISH HER2 determi...