In order to identify genes associated with poor outcome of ER+ berast cancers treated with antiestrogens, we had performed whole transcriptome analysis on RNA extracted from 58 ER+ breast cancers from patients treated with long-term letrozole for a median of 7.2 months (Supplementary Table 1; cohort of Guerrero-Zotano et al.17). This evidence concerns the gene ESR1 and breast cancer.