Key Inclusion Criteria:

          -  Histologically or cytologically confirmed breast cancer with evidence of metastatic or
             locally advanced disease not amenable to resection or radiation therapy with curative
             intent and who have progressed during treatment with at least one prior hormonal
             therapy

               -  Phase 1b Dose Escalation - Individuals may have had unlimited prior hormonal
                  therapy and a total of 2 prior chemotherapy regimens (adjuvant chemotherapy is
                  considered 1 regimen). Individuals may have progressed on fulvestrant or
                  exemestane.

               -  Randomized Phase 2 Dose Expansion - Individuals may have disease progression
                  during treatment or within 12 months of completion of endocrine therapy
                  (tamoxifen, and/or AI) in the adjuvant setting, or disease progression during
                  treatment with endocrine therapy (tamoxifen, AI or CDK4/6 inhibitor plus AI) for
                  advanced/metastatic disease. Individuals may have had unlimited prior hormonal
                  therapy, but must be naive to fulvestrant in the metastatic setting. A total of 2
                  prior chemotherapies are allowed, however, only one for metastatic disease is
                  permitted.

          -  Documentation of ER positive (≥ 1% positive stained cells by local standards) based on
             the most recent tumor biopsy, unless bone-only disease

          -  Documented HER2-negative tumor based on local testing on most recent tumor biopsy
             (immunohistochemistry score 0/1+ or negative by in situ hybridization HER2/CP17 ratio
             < 2 or for single probe assessment HER2 copy number < 4)

          -  Post-, pre- or peri-menopausal women considered to be in the post-menopausal state as
             defined by one of the following:

               -  Age ≥ 60 years

               -  Age < 60 years and cessation of regular menses for at least 12 consecutive months
                  in the absence of chemotherapy, tamoxifen, toremifene, or ovarian suppression and
                  serum estradiol and follicle-stimulating hormone (FSH) level within the
                  post-menopausal range

               -  Prior bilateral oophorectomy

               -  Pre-/peri-menopausal women can be enrolled if amenable to be treated with the
                  luteinizing-hormone releasing hormone (LHRH) agonist, goserelin. Individuals must
                  have commenced treatment with goserelin or an alternative LHRH agonist at least 4
                  weeks prior to Cycle 1 Day 1 (C1D1). If individuals have received an alternative
                  LHRH agonist prior to study entry, they must switch to goserelin on or before
                  Cycle 1 Day 1 (C1D1) for the duration of the study

          -  Measurable disease defined per RECIST v. 1.1, or bone-only disease must have a lytic
             or mixed lytic blastic lesion that can be accurately assessed by computed tomography
             (CT) or magnetic resonance imaging (MRI). Individuals with bone-only disease and
             blastic-only metastases are not eligible

          -  All acute toxic effects of any prior antitumor therapy resolved to Grade ≤ 1 before
             the start of study drug dosing (with the exception of alopecia [Grade 1 or 2
             permitted] and neurotoxicity [Grade 1 or 2 permitted])

          -  Eastern Cooperative Oncology Group (ECOG) Performance Status of ≤ 1

          -  Life expectancy of ≥ 3 months, in the opinion of the investigator

          -  Adequate organ function defined as follows:

               -  Hematologic: Platelets ≥ 100 x 10^9/L; Hemoglobin ≥ 9.0 g/dL; absolute neutrophil
                  count (ANC) ≥ 1.5 x 10^9/L (without platelet transfusion or any granulocytic
                  growth factors within previous 7 days of the hematologic laboratory values
                  obtained at screening visit)

               -  Hepatic: aspartate transaminase (AST) / alanine transaminase (ALT) ≤ 2.5 x upper
                  limit of normal (ULN) (if liver metastases are present, ≤ 5 x ULN); total or
                  conjugated bilirubin ≤ 1.5 x ULN

               -  Renal: Serum Creatinine ≤ 1.5 x ULN or creatinine clearance (CrCl) ≥ 60 mL/min as
                  calculated by the Cockroft-Gault method

          -  Coagulation: International Normalized Ratio (INR) ≤ 1.2

          -  Negative serum pregnancy test

          -  Females of childbearing potential who engage in heterosexual intercourse must agree to
             use protocol specified method(s) of contraception as described in the study protocol

          -  Females who are nursing must agree to discontinue nursing before the first dose of
             GS-5829

          -  Able and willing to provide written informed consent to participate in the study

        Key Exclusion Criteria:

          -  History or evidence of clinically significant disorder, condition, or disease that, in
             the opinion of the investigator or the Gilead medical monitor would pose a risk to
             individual safety or interfere with the study evaluations, procedures, or completion

          -  Known brain metastasis or leptomeningeal disease (Note: if treated and stable at least
             6 months prior to enrollment, individual is eligible).

          -  Uncontrolled intercurrent illness including, but not limited to, active uncontrolled
             infection, active or chronic bleeding event within 28 days prior to C1D1, uncontrolled
             cardiac arrhythmia, or psychiatric illness/social situation that would limit
             compliance with study requirements as judged by treating physician

          -  Myocardial infarction, symptomatic congestive heart failure (New York Heart
             Association Classification > Class II), unstable angina, or serious uncontrolled
             cardiac arrhythmia within the last 6 months of C1D1

          -  Major surgery, defined as any surgical procedure that involves general anesthesia and
             a significant incision (i.e., larger than what is required for placement of central
             venous access, percutaneous feeding tube) within 28 days of C1D1

          -  Impairment of gastrointestinal (GI) function or GI disease that may significantly
             alter the absorption of GS-5829, including any unresolved nausea, vomiting, or
             diarrhea that is Common Terminology Criteria for Adverse Events (CTCAE) Grade > 1

          -  Minor surgical procedure(s) within 7 days of enrollment or randomization, or not yet
             recovered from prior surgery (placement of central venous access device, fine needle
             aspiration, or endoscopic biliary stent ≥ 1 day before enrollment or randomization is
             acceptable)

          -  History of a concurrent or second malignancy, except for: adequately treated local
             basal cell or squamous cell carcinoma of the skin; cervical carcinoma in situ;
             superficial bladder cancer; adequately treated Stage 1 or 2 cancer currently in
             complete remission; any other cancer that has been in complete remission for ≥ 5 years

          -  Anti-tumor therapy (chemotherapy, chemoradiation, radiation, antibody therapy,
             molecular targeted therapy) within 21 days or 5 half-lives whichever is longer, of
             C1D1 (6 weeks for nitrosoureas, mitomycin C, or molecular agents with t1/2 > 10 days);
             5 half-lives of any investigational drug; concurrent use of goserelin for
             pre-/peri-menopausal breast cancer and exemestane or fulvestrant per the protocol are
             permitted

          -  History of long QT syndrome or whose corrected QT interval (QTc) measured (Fridericia
             method) at screening is prolonged (> 470 ms). Individuals who screen fail due to this
             criterion are not eligible to be re-screened

          -  Prior exposure to any bromodomain (BET) inhibitors

          -  Known hypersensitivity to the study drugs (GS 5829, fulvestrant or exemestane), the
             metabolites, or formulation excipients

          -  Immunotherapy within 6 months of C1D1

          -  Evidence of bleeding diathesis or clinically significant bleeding, within 28 days of
             C1D1 or history of hemoptysis of > 2.5 mL/1 teaspoon within 6 months of C1D1

          -  Anticoagulation/antiplatelet therapy within 7 days of C1D1, including acetylsalicylic
             acid, low molecular weight heparin, or warfarin

          -  Known human immunodeficiency virus (HIV) infection

          -  Hepatitis B surface Antigen (HBsAg) positive

          -  Hepatitis C virus (HCV) antibody positive with HCV RNA positive

          -  Use of moderate/strong cytochrome P450 (CYP)3A4 inhibitors or moderate/strong CYP3A4
             inducers within 2 weeks prior to C1D1

          -  History of high grade esophageal or gastric varices

        Note: Other protocol defined Inclusion/Exclusion criteria may apply.