Intrinsic subtype stratified by the proliferation index. Tumors were given an 'intrinsic' subtype assignment based on the minimal 37-gene quantitative reverse-transcription-PCR classifier (Figure 1b). Patients were classified as having Luminal (estrogen receptor (ER)-positive) or HER2/Basal (ER-negative) subtypes. In order to have groups of similar size and because the subtypes largely follow ER status, tumors in the HER2 and Basal-like groups (both ER-negative) were combined. Continuous expression data for the proliferation meta-gene (log2 average of the 14 selected markers) were used in a Cox regression model to determine the probability of relapse over time. Differences in relapse for low (green), medium (red), and high (blue) expression are shown as tertiles in the Kaplan-Meier plots. Stratification by proliferation added information for relapse in the Luminal subtype (P = 0.00039) but not the ER-negative subtypes (P = 0.74).