Both 2-OHE1 and 16α-OHE1 are estrogen-like compounds, however, the 16α-OHE1 metabolite is a potent estrogenic molecule that activates the ER and induces proliferation of cultured breast cancer cells by 40%, whereas the 2-OHE1 metabolite has very little estrogen receptor binding affinity (< 0.1% compared to estradiol) [7], and has been shown to decrease cell proliferation by 20 to 30% in cultured breast cancer cell lines [8,9]. The gene discussed is ESR1; the disease is breast carcinoma.