Cohort 1: - Have histologically confirmed Stage I to III Estrogen Receptor positive (ER+), human epidermal growth factor receptor 2 negative (HER2-) invasive breast carcinoma with Ki-67 at least 10% - Be willing and able to provide pre- and on-treatment tumor samples. - Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1. - Be able to swallow capsules or tablets. - Be premenopausal women. - If of childbearing potential must use 1 highly effective method of non-hormonal contraception while receiving study treatment and for the duration specified in protocol. - Have adequate organ function. Cohort 2: - Have a diagnosis of ER+, HER2- early-stage, resected, invasive breast cancer without evidence of distant metastasis - Have undergone definitive loco-regional therapy. - Have received at least 4.5 years of any adjuvant endocrine therapy (ET), or at least 2 years of adjuvant ET with no additional ovarian suppression planned. - Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1. - Be able to swallow capsules or tablets. - Be premenopausal women - If of childbearing potential must use 1 highly effective method of non-hormonal contraception while receiving study treatment and for the duration specified in protocol. - Have adequate organ function.

Cohort 1: - Have bilateral invasive metastatic, occult primary, or inflammatory breast cancer. - Have had prior bilateral oophorectomy or ovarian ablation. - Have a serious medical condition - Had major surgery within 28 days prior to randomization. - Have a history of other cancer (except non melanoma skin cancer, Stage I uterine cancer, or carcinoma in situ of the cervix or other in situ cancer), unless in complete remission with no therapy for a minimum of 1 year. - Plan to receive concurrent neoadjuvant therapy with any other non-protocol anticancer therapy. - Have had any prior therapy for an invasive or non-invasive breast cancer. - Have had prior radiotherapy to the ipsilateral chest wall for any malignancy. - Have received prior anti-estrogen therapy, including for osteoporosis or prevention of breast cancer. - Have had prior treatment with any Gonadotropin-releasing hormone (GnRH) agonist within 12 months prior to randomization. - Receiving current exogenous reproductive hormone therapy Cohort 2: - Have ovarian cyst(s) greater than (>) 1 centimeter (cm) at screening. - Have metastatic occult primary, or inflammatory breast cancer. - Have had prior bilateral oophorectomy or ovarian ablation. - Have a serious medical condition - Had major surgery within 28 days prior to randomization. - Have a history of other cancer (except non melanoma skin cancer or carcinoma in situ of the cervix or other in situ cancer), unless in complete remission with no therapy for a minimum of 1 year. - Completed or discontinued prior adjuvant ET >6 months prior to screening. - Have received prior therapy with any selective estrogen receptor degrader (SERD). - Receiving current exogenous reproductive hormone therapy.