We will perform a randomized, open-label, single-institution study. It will compare the efficacy of three different endocrine treatment regimens (Anastrozole +/- Fulvestrant or Tamoxifen) in changing proliferation-index and inducing apoptosis during a 2-6 week pre-operative treatment period in breast cancer patients. These results will be correlated to gene expression profiles, phosphorylation status of the ER, SNPs in CYP450 sequences, tamoxifen metabolite concentrations, changes in estrogen serum levels and protein expression patterns.