To Study the Effect of Inavolisib in Combination With Fulvestrant in Participants With Breast Cancer

Who May Qualify: - Documented ER +/ HER2- tumor according to American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines - Disease progression after or during treatment with a combination of CDK4/6i and endocrine therapy: \<= 1 prior lines of systemic therapy in the locally advanced (recurrent or progressed) or metastatic setting - Measurable or evaluable disease per Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST v1.1) - Participants for whom endocrine-based therapy is recommended and treatment with cytotoxic chemotherapy is not indicated at time of entry into the study, as per national or local treatment guidelines - Confirmation of biomarker eligibility: presence of \>= 1 study-eligible PIK3CA mutation - Life expectancy of \> 6 months - Ability, in the investigator's judgment, and willingness to comply with all study -related procedures, including completion of patient-reported outcomes Who Should NOT Join This Trial: - Metaplastic breast cancer - Prior treatment with chemotherapy in the recurrent locally advanced/metastatic setting - Type 2 diabetes requiring ongoing systemic treatment at the time of study entry; or any history of Type 1 diabetes - Prior treatment with PI3K/Akt/mTOR inhibitors in the recurrent locally advanced/metastatic setting - Requirement for daily supplemental oxygen - Symptomatic active lung disease, including pneumonitis Always talk to your doctor about whether this trial is right for you.