In the study stage I-II, HER2 negative, ER+, female breast cancer patients received eribulin mesylate (1.4 mg/m2, i.v., day 1 and 8) and capecitabine (900 mg/m2, oral BID, days 1–14) on a 21 day cycle, for four cycles. Here, ESR1 is linked to breast cancer.