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Background: Histologic grade in breast cancer provides clinically important prognostic information. However, 30% – 60% of tumors are classifi ed as histologic grade 2. This grade is associated with an intermediate risk of recurrence and is thus not informative for clinical decision making. We examined whether histologic grade was associated with gene expression profi les of breast cancers and whether such profi les could be used to improve histologic grading. Methods: We analyzed microarray data from 189 invasive breast carcinomas and from three published gene expression datasets from breast carcinomas. We identifi ed differentially expressed genes in a training set of 64 estrogen receptor (ER) – positive tumor samples by comparing expression profi les between histologic grade 3 tumors and histologic grade 1 tumors and used the expression of these genes to defi ne the gene expression grade index. Data from 597 independent tumors were used to evaluate the association between relapse-free survival and the gene expression grade index in a Kaplan – Meier analysis. All statistical tests were two-sided. Results: We identifi ed 97 genes in our training set that were associated with histologic grade; most of these genes were involved in cell cycle regulation and proliferation. In validation datasets, the gene expression grade index was strongly associated with histologic grade 1 and 3 status; however, among histologic grade 2 tumors, the index spanned the values for histologic grade 1 – 3 tumors. Among patients with histologic grade 2 tumors, a high gene expression grade index was associated with a higher risk of recurrence than a low gene expression grade index (hazard ratio = 3.61, 95% confi dence interval = 2.25 to 5.78; P