 This is an open-label, multicenter, Phase I study designed to evaluate the safety, tolerability, and pharmacokinetics of GDC-0077 administered orally as a single agent in participants with locally advanced or metastatic Phosphatidylinositol-4,5-Bisphosphate 3-Kinase Catalytic Subunit Alpha PIK3CA-mutant solid tumors, including breast cancer, and in combination with standard-of-care endocrine and targeted therapies for the treatment of locally advanced or metastatic PIK3CA-mutant hormone receptor-positive HRhuman epidermal growth factor receptor 2 negative HER2- breast cancer. Participants will be enrolled in two stages: a dose-escalation stage Stage I and an expansion stage Stage II. Participants will be assigned to one of four regimens: GDC-0077 as a single agent Arm A, GDC-0077 in combination with palbociclib and letrozole Arm B, GDC-0077 in combination with letrozole Arm C, or GDC-0077 in combination with fulvestrant Arm D. 