ESR1 and breast cancer: Regarding tumor characteristics study groups differed in frequency of secondary breast cancer (8.5% (BCS) vs. 40.7% (M) vs. 17.4% (MIBR)), estrogen receptor positive tumors (82.6% (BCS) vs. 82.6% (M) vs. 63.7% (MIBR)), tumor grading (e.g. G3: 21.7% (BCS) vs. 19.0% (M) vs. 41.9% (MIBR)), tumor size (e.g. (y)pT1: 45.8% (BCS) vs. 14.8% (M) vs. 39.3% (MIBR) and (y)pT3: 4.2% (BCS) vs. 25.9% (M) vs. 6.0% (MIBR)) and nodal positivity (≥ (y)pN1: 16.5% (BCS) vs. 40.7% (M) vs. 25.0% (MIBR)) (see Additional file 1).