Among these estrogen-dependent conditions are estrogen-sensitive breast tumors. In these cancers, a certain enzyme, aromatase, is highly expressed and is responsible for converting androgen substrates into estrogens. With this knowledge, researchers have developed aromatase-inhibiting compounds shown to be beneficial to patients with this tumor type. Aromatase inhibitors (AIs) act to reduce circulating and localized estrogen levels and are currently marketed for long-term therapy in early to late stage breast cancer. Resulting from the culmination of more than two decades of drug development efforts, currently marketed AIs are third generation products and are generally accepted as safe with acceptable side effects.