This then helps us build on previously published work from our lab as well that of other labs demonstrating that ESR1 fusions have unique activity and frequency in metastatic ER + breast cancer tissue [24–26]. This demonstrates the value and need to collect novel metastatic tissue samples, as they may reveal novel mechanisms of resistance that are heterogenous between tissue biopsies and only evident in late-stage disease. This evidence concerns the gene ESR1 and breast carcinoma.