A Study of Imlunestrant (LY3484356) in Premenopausal Women With Estrogen Receptor-Positive (ER+) Human Epidermal Growth Factor Receptor 2 Negative (HER2-) Early Breast Cancer

Inclusion Criteria: 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. Exclusion Criteria: 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.