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

This study is testing a combination of two drugs — inavolisib (a targeted therapy that blocks a cancer-driving pathway called PI3K) and fulvestrant (a hormone therapy) — for women with hormone receptor-positive, HER2-negative breast cancer that has grown back or spread after prior treatment. **You may be eligible if...** - You have estrogen receptor-positive (ER+), HER2-negative breast cancer that has advanced or spread - Your cancer has a specific mutation in the PIK3CA gene (confirmed by testing) - Your cancer progressed during or after prior CDK4/6 inhibitor therapy combined with hormone therapy - You have had at most one prior treatment regimen for advanced disease - Hormone-based therapy is still the recommended approach for your cancer **You may NOT be eligible if...** - Your tumor does not have the required PIK3CA mutation - You have HER2-positive breast cancer - You have uncontrolled diabetes or other serious conditions that would be worsened by PI3K inhibitors - You are pregnant or breastfeeding - You have received more than one prior treatment line for your advanced/metastatic disease Talk to your doctor to see if this trial is right for you.