Tanaproget (WAY-166989) is a nonsteroidal progesterone receptor agonist. It has been in Phase-II clinical trials as an oral contraceptive. The compound demonstrated a positive preclinical pharmacological profile in the treatment of endometriosis. The level of progesterone receptors in breast tumours can be used to guide the selection of endocrine therapies for breast cancer patients. Radiolabeled analogues of tanaproget have diagnostic potential as PET imaging agents for breast cancer.