A Study of LY3484356 in Participants With Advanced or Metastatic Breast Cancer or Endometrial Cancer

Inclusion Criteria: All study parts: * Participants must be willing to provide adequate archival tissue sample * Participants must be willing to use highly effective birth control * Participants must have adequate organ function * Participants must be able to swallow capsules Dose escalation- Participants must have one of the following: * Parts A and B: ER+ HER2- breast cancer with evidence of locally advanced unresectable or metastatic disease who have had the following: * Part A: may have had up to 1 prior regimen of any kind for in the advanced/metastatic setting and no prior cyclin-dependent kinase 4/6 (CDK4/6) inhibitor therapy. * Part B: may have had up to 2 prior regimens, no more than 1 of which may be endocrine therapy in the advanced/metastatic setting, and must have received a prior CDK4/6 inhibitor * Cohort E4: No prior everolimus. * Cohort E5: No prior alpelisib and must have a phosphatidylinositol 3-kinase catalytic α (PIK3Cα) mutation as determined by local testing. * Part C: ER+, human epidermal growth factor receptor 2 positive (HER2+) breast cancer with evidence of locally advanced unresectable or metastatic disease who have had at least 2 HER2-directed therapies in any setting. * Part D: ER+, EEC that has progressed after platinum containing chemotherapy and no prior fulvestrant or aromatase inhibitor therapy. * Part E: ER+ and HER2+ breast cancer with evidence of locally advanced, unresectable, or metastatic disease. * Part E: Participants must have re…