Low-grade serous carcinoma of the ovary (LGSOC) is a rare ovarian cancer often diagnosed in younger women. These cancers generally grow slowly; however, they spread early and are usually incurable. LGSOC tumours frequently express estrogen receptors (ER) and progesterone receptors (PR) on the surface of the cancer cells. Drugs that target the ER (called anti-estrogens therapies, or “AET”) are used frequently in the treatment of LGSOC. Surprisingly little is known about the way AETs work in LGSOC and how effective they are for the treatment of these patients; we urgently need to understand this. We intend to intensively study 80 patients diagnosed with LGSOC. We will classify these cancers according to their ER/PR expression and their genetic makeup. This information will be used to validate the results of one of our prior studies showing that ER/PR expression influences the outcome of these cancers. We intend to study LGSOC models we have developed in mice that express ER. We will treat these mice with AETs to determine which genes the ER is regulating and how these genes might affect the growth of these cancers. We will in turn use this information to test LGSOC tissues obtained from human patients to see if the same molecular changes are found. This research will help identify which patients would benefit from AET and help us design future drug studies to combine AET with other effective new drugs.