The researchers hypothesize that the cyclinD1-interactome can be used to orient the use of fulvestrant in premenopausal and postmenopausal women. To test this hypothesis, the researchers propose a pre-surgical randomized clinical trial of tamoxifen vs fulvestrant in the window between breast cancer diagnosis on core biopsy and definitive surgery. Women with ER/cyclinD1 positive tumors will be eligible. Response to tamoxifen or fulvestrant will be evaluated using standard proliferation index as well as gene expression signatures obtained in pre-clinical models of tamoxifen resistance and sensitivity to fulvestrant. In addition, the researchers propose to use cutting edge new technology allowing ex-vivo expansion of primary culture from only a few cancer cells obtained by fine needle biopsy. The researchers propose to compare the response of these primary cells to patient response. If successful, the impact of this work can support the expansion of use of fulvestrant to not only postmenopausal women but premenopausal women as well. In addition, it may serve as a proof of principle to maximize the use of biopsy material to predict treatment response