ESR1 and metastatic neoplasm: Of these, the two ESR1 fusions identified from endocrine therapy-refractory, ER+ metastatic disease, ESR1-e6>YAP1 and ESR1-e6>PCDH11X, drove endocrine therapy resistant proliferation in experimental models, while ESR1-e6>NOP2 from an endocrine therapy naïve primary tumor did not.