ESR1 and breast cancer: In contrast, elacestrant which shows improved bioavailability and pharmacokinetic properties, is capable of targeting both residual wild-type and mutated ESR116,34, a finding supported clinically in the recent phase I trial of elacestrant where anti-tumour activity was observed in heavily pre-treated patients with metastatic BC including patients whose tumours harboured ESR1 mutations as well as in patients who had progressed on fulvestrant27.