This is an open-label, single arm, phase II trial to evaluate the efficacy and safety of 500mg Fulvestrant (Faslodex®) in ESR1 mutated postmenopausal women with hormone receptor positive, HER2 negative locally advanced or metastatic breast cancer after previous aromatase inhibitor therapy. Fifty patients will be enrolled and treated with 500 mg Fulvestrant until disease progression or study closed.