This is a study of LY3484356 in Chinese participants with advanced breast cancer. Participants must have breast cancer that is estrogen receptor positive (ER+), HER2 negative (HER2-). The purpose of this study is to measure how much LY3484356 gets into the bloodstream and how long it takes the body to remove it. The safety and effectiveness of LY3484356 will also be studied. Participation could last up to 28 months.

Inclusion Criteria:

• Native Chinese participants must be of an acceptable age to provide informed consent
• Have locally advanced (not amenable to curative treatment by surgery) or metastatic disease and be an appropriate candidate for experimental therapy in the judgment of the investigator, after available standard therapies have ceased to provide clinical benefit
• Have a diagnosis of ER+, HER2- breast cancer
• Female participants have postmenopausal status due either surgical/natural menopause or ovarian suppression
• If postmenopausal status is due to ovarian suppression, participants must have a negative serum pregnancy test and agree to use highly effective, medically approved precautions to prevent pregnancy
• Have a performance status less than or equal to (≤)1 on the Eastern Cooperative Oncology Group (ECOG) scale
• Have adequate organ function
• Must be able to swallow capsules/tablets

Exclusion Criteria:

• Have symptomatic central nervous system (CNS) metastasis and/or carcinomatous meningitis

• Have a serious concomitant systemic disorder

  • Human immunodeficiency virus (HIV) positive patients are excluded unless they are well controlled on highly active antiretroviral therapy with no evidence of autoimmune deficiency syndrome-defining opportunistic infections within the last 2 years, and cluster of differentiation 4 (CD4) count greater than (>)350 cells/microliter (μL)
  • Active hepatitis B or C virus infection
  • Severe renal impairment, interstitial lung disease, severe dyspnea at rest or requiring oxygen therapy, history of major surgical resection involving the stomach or small bowel, preexisting Crohn's disease or ulcerative colitis or a preexisting chronic condition resulting in clinically significant diarrhea

• Have visceral crisis

• Have a serious cardiac condition

• Have an acute leukemia or other relevant cancers

• Females who are pregnant or lactating

• Known allergic reaction against any of the components of the study drug