Inclusion Criteria:

          -  Histological or cytological confirmation of estrogen receptor-positive (ER+) invasive
             carcinoma of the breast. ER+, HER2- negative invasive carcinoma of the breast with
             evaluable sample for BCL-2 IHC value at the time of screening. Participants who were
             originally diagnosed with HER2-positive breast cancer that converted to HER2-negative
             MBC are not eligible.

          -  Evidence of metastatic or locally advanced disease not amenable to surgical or local
             therapy with curative intent.

          -  Be postmenopausal or pre- or perimenopausal women amenable to being treated with the
             luteinizing hormone-releasing hormone (LHRH) agonist goserelin.

          -  Participants must not have received more than two prior lines of hormonal therapy in
             the locally advanced or metastatic setting. In addition, at least one line of
             treatment must be a CDK4/6i AND participants must have experienced disease recurrence
             or progression during or after CDK4/6i therapy, which must have been administered for
             a minimum of 8 weeks prior to progression.

          -  Participants for whom endocrine therapy (e.g., fulvestrant) is recommended and
             treatment with cytotoxic chemotherapy is not indicated at the time of entry into the
             study, as per national or local treatment guidelines.

          -  Women of childbearing potential (i.e., not postmenopausal for at least 12 months or
             surgically sterile) must have a negative serum pregnancy test result at screening,
             within 14 days prior to the first study drug administration.

          -  For women of childbearing potential: agreement to remain abstinent (refrain from
             heterosexual intercourse) or use non-hormonal contraceptive methods with a failure
             rate of <1% per year during the treatment period and for up to 2 years after the last
             dose of study drug (or based on the local prescribing information for fulvestrant).
             Women must refrain from donating eggs during this same period.

          -  Willing to provide tumor biopsy sample.

          -  Have at least one measurable lesion via RECIST v1.1.

          -  Have an Eastern Cooperative Oncology Group (ECOG) Performance Score of 0-1.

          -  Have adequate organ and marrow function.

          -  Have a life expectancy > 3 months.

          -  To full fill the coagulation requirements for patient with or without therapeutic
             anticoagulation.

        Exclusion criteria:

          -  Prior treatment with fulvestrant or other selective estrogen receptor degraders
             (SERDs), venetoclax, or any agent whose mechanism of action is to inhibit BCL-2.

          -  Pregnant, lactating, or intending to become pregnant during the study.

          -  Known untreated or active Central Nervous System (CNS) metastases (progressing or
             requiring anticonvulsants or corticosteroids for symptomatic control.

          -  Prior chemotherapy in the locally advanced or metastatic setting regardless of the
             duration of the treatment.

          -  Any anti-cancer therapy received within 21 days of the first dose of study drug,
             including chemotherapy, radiotherapy, hormonal therapy, immunotherapy, antineoplastic
             vaccines, or other investigational therapy. (Radiotherapy with palliative intent to
             non-target sites is allowed).

          -  Concurrent radiotherapy to any site or prior radiotherapy within 21 days of Cycle 1
             Day 1 or previous radiotherapy to the target lesion sites (the sites that are to be
             followed for determination of a response) or prior radiotherapy to > 25% of bone
             marrow.

          -  Current severe, uncontrolled, systemic disease (e.g., clinically significant
             cardiovascular, pulmonary, metabolic or infectious disease.

          -  Any major surgery within 28 days of the first dose of study drug or anticipation of
             the need for major surgery during the course of study treatment.

          -  Consumption of one or more of the following within 3 days prior to the first dose of
             study drug: Grapefruit or grapefruit products; Seville oranges including marmalade
             containing Seville oranges; Star fruit (carambola).

          -  Administration within 7 days prior first dose of study treatment of Steroid therapy
             for anti-neoplastic intent, Strong or moderate CYP3A inhibitors or Strong or moderate
             CYP3A inducers.

          -  Need for current chronic corticosteroid therapy (> 10 mg of prednisone per day or an
             equivalent dose of other anti-inflammatory corticosteroids).

          -  Known infection with (human immunodeficiency virus) HIV or human T-cell leukemia virus
             1.

          -  Known active bacterial, viral, fungal, mycobacterial, parasitic, or other infection
             (excluding fungal infections of nail beds) at study enrollment, or any major episode
             of infection requiring treatment with IV antibiotics or hospitalization (relating to
             the completion of the course of antibiotics) within 4 weeks prior to Cycle 1 Day).

          -  Positive test results for hepatitis B core antibody (HBcAb) or hepatitis C virus (HCV)
             antibody at screening. Participants who are positive for HCV antibody must be negative
             for HCV by PCR to be eligible for study participation. Participants with a past or
             resolved hepatitis B virus (HBV) infection (defined as having a positive total HBcAb
             and negative hepatitis B surface antigen [HbsAg]) may be included if HBV DNA is
             undetectable. These participants must be willing to undergo monthly DNA testing.

          -  Participants who have a positive HCV antibody test are eligible for the study if a PCR
             assay is negative for HCV RNA.

          -  History of other malignancies within the past 5 years except for treated skin basal
             cell carcinoma, squamous cell carcinoma, non-malignant melanoma <= 1.0 mm without
             ulceration, localized thyroid cancer, or cervical carcinoma in-situ.

          -  Administration of a live, attenuated vaccine within 4 weeks prior to initiation of
             study treatment or anticipation of need for such a vaccine during the study.

          -  Cardiopulmonary dysfunction.

          -  Other medical or psychiatric conditions that, in the opinion of the investigatory, may
             interfere with the participant's participation in the study.

          -  Inability or unwillingness to swallow pills or receive intramuscular (IM) injections.

          -  History of malabsorption syndrome or other condition that would interfere with enteral
             absorption.

          -  History of inflammatory bowel disease (e.g., Crohn's disease or ulcerative colitis) or
             active bowel inflammation (e.g., diverticulitis).

          -  Concurrent hormone replacement therapy.

          -  Inability to comply with study and follow-up procedures.

          -  History or active cardiopulmonary dysfunction.

          -  Known hypersensitivity to any of the study medications (fulvestrant, venetoclax) or to
             any of the excipients.