Steroids and related compounds are important in disease development and prevention, via steroid receptor-mediated and receptor-independent mechanisms. Interaction of endogenous oestrogens with the oestrogen receptor has unfortunate mitogenic effects in breast cancer. However, dietary consumption of non-steroidal weak oestrogens, such as the soy isoflavone phytoestrogens genistein and diadzein, is associated with a decreased breast cancer risk. This may arise in part from the suboptimal configuration induced in the transactivation helix of oestrogen receptor-beta.