There are two possible explanations: First, the E-MTAB-365 ER-positive sub-cohort has relatively lower relapse rate compared to the exploration HRPBC cohort and GSE4922 ER-positive cohort (23.7% in E-MTAB-365 vs. 38.7% in exploration HRPBC and 36.0% in GSE4922) which may have hindered the discerning power of survival analysis; Second, in microarray datasets, gene expressions were analyzed using total tissue mRNA, and mRNA from cell types other than breast cancer cells may interfere with the expression of certain genes. Here, ESR1 is linked to breast cancer.