To date, no prospective, randomized, placebo-controlled clinical trials have been conducted to test the hypothesis that exposure to diindolylmethane (DIM), in combination with tamoxifen (TAM), can significantly modify breast cancer risk. The DIME Study tests the novel hypothesis that DIM will demonstrate an enhanced efficacy of TAM by modulating the breast parenchyma to a ‘low’ density, ‘low risk’ state and alternatively by favorably modulating steroid hormone metabolism. In fact, DIM has been shown to act on breast cancer risk through a variety of mechanisms, and as such, core breast biopsy tissue samples and blood for alternate hypotheses testing (e.g., COX-2, prolactin receptor, proliferation assays, AhR, inflammatory markers, oxidative stress, etc) are collected in the context of this trial. The recruitment portion of the study ended in September 2015. Study results will be available as of late 2016. This study was funded by the National Cancer Institute.