ESR1 and breast cancer: Using pre-treatment gene expression data from an independent validation cohort of stage I-III breast cancer patients treated with neo-adjuvant chemotherapy41, we also observed higher levels of the CD8 + T-cell signature and IFN-γ signature in breast tumors with 0%, 1–9% and 10–50% ER expression, as compared to tumors with ER expression >50% (Supplementary Fig. 4A–D).