ESR1 score cut-offs using training set and the GEICAM/9906 testing set. The ESR1 score is provided as a qualitative call of high, intermediate, or low. The cut-offs were based on the continuous expression of ESR1 across prototype samples in the training set. Each circle on the box plot represents an individual sample that is color coded according to IHC status. The cut-points between high, intermediate, and low classes were individually derived from the training set samples (A). Data from ESR1 gene expression over the GEICAM 9906 samples (B) are plotted on the same scale as the training set. Samples are colored according to ER IHC positivity (red) or negativity (blue) determined at a central facility.