This multicenter, randomized, single-blind, placebo-controlled, two arm study will evaluate the efficacy and safety of paclitaxel with GDC-0941 versus paclitaxel with placebo in participants with locally recurrent or metastatic breast cancer.

Inclusion Criteria:

• Histologically or cytologically confirmed adenocarcinoma of the breast, with measurable or non-measurable locally recurrent or metastatic disease
• Human epidermal growth factor receptor 2 (HER2)-negative and hormone receptor (HR) (estrogen receptor and/or progesterone receptor)-positive disease as defined by local guidelines
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
• Adequate hematologic and end organ function
• Women of childbearing potential must agree to remain abstinent or to use two adequate methods of contraception, including at least one method with a failure rate of less than (<) 1 percent (%) per year, during the treatment period and for at least 30 days after the last dose of study treatment or 6 months after discontinuation of paclitaxel, whichever is longer

Exclusion Criteria:

• Prior non-capecitabine chemotherapy for locally recurrent or metastatic disease
• Prior treatment with a phosphoinositide 3-kinase (PI3K) inhibitor for advanced or metastatic breast cancer
• History of intolerance to a taxane-containing therapy
• History of clinically significant cardiac or pulmonary dysfunction
• History of malabsorption syndrome or other condition that would interfere with enteral absorption
• Clinically significant history of liver disease
• Active autoimmune disease or active inflammatory disease
• Immunocompromised status due to current known active infection with human immunodeficiency virus (HIV) or due to the use of immunosuppressive therapies for other conditions
• Need for current chronic corticosteroid therapy
• Pregnant, lactating, or breastfeeding women
• Current severe, uncontrolled systemic disease
• Known untreated or active central nervous system (CNS) metastases