ESR1 and breast cancer: Our group previously demonstrated in ER+/human epidermal growth factor receptor 2‐negative(HER2−) breast cancer patients that total levels of IGF‐1R did not predict resistance to adjuvant tamoxifen.20 In a small study on invasive primary and recurrent tamoxifen‐resistant tumors of the same patient, there was no evidence of increased IGF‐1R expression.21 Whether IGF‐1R phosphorylation and concurrent activation of the PI3K and/or MAPK pathway are indicative for response to adjuvant endocrine therapy in breast cancer patients remains elusive.