This is an open-label, dose-finding, safety, pharmacokinetics (PK), and evidence-of-activity study of GDC-0927 in postmenopausal women with locally advanced or metastatic Estrogen Receptor Positive (ER+) Human Epidermal Growth Factor Receptor 2 (HER2) breast cancer. The study will be conducted in two parts: Dose escalation and Dose expansion. During dose escalation, GDC-0927 will be administered orally as a single dose on Day -7 for PK evaluation during the lead-in period. Depending on safety and tolerability, participants will be assigned sequentially to escalating doses of GDC-0927 using standard 3+3 design. During dose expansion, there will be no PK week lead-in period. All participants will be treated until disease progression, unacceptable toxicity, participant withdrawal of consent or study termination.