Letrozole is an aromatase inhibitor, which can inhibit the conversion of androgens secreted by the adrenal cortex into estrogen, thus reducing the level of estrogen in the body, and has certain therapeutic effects on hormone-dependent breast cancer. It is mainly used for the treatment of postmenopausal patients with advanced breast cancer who are ineffective in anti-estrogen therapy. Exemestane is an irreversible steroidal aromatase inactivator, both of which are for the treatment of hormone-dependent postmenopausal breast cancer patients, but exemestane is an alternative to tamoxifen for the treatment of postmenopausal breast cancer patients whose disease is still progressing after tamoxifen administration.