Inclusion Criteria:

          -  Confirmed diagnosis of HR-positive, HER2-negative breast cancer, not amenable to
             curative therapy

          -  Pre- or perimenopausal women can be enrolled if amenable to be treated with goserelin

          -  Patients must satisfy 1 of the following criteria for prior therapy:

               -  Progressed during treatment or within 12 months of completion of adjuvant therapy
                  with an aromatase inhibitor or tamoxifen

               -  Progressed during treatment or within 2 months after the end of prior aromatase
                  inhibitor therapy for advanced/metastatic breast cancer, or prior endocrine
                  therapy for advanced/metastatic breast cancer

               -  Received ≤ 2 chemotherapy regimens (Part 1) or ≤ 1 chemotherapy regimen (Part 2)
                  for advanced/metastatic disease

          -  For Part 1, evaluable or measurable disease (bone only disease eligible for Part 1
             only)

          -  For Part 2, measurable disease as defined by RECIST, Version 1.1

          -  ECOG performance status 0 to 1

          -  Adequate organ function

        Exclusion Criteria:

          -  For Part 1, prior treatment with fulvestrant

          -  For Part 2, prior treatment with any CDK inhibitor or fulvestrant

          -  Active uncontrolled/symptomatic CNS metastases, carcinomatous meningitis, or
             leptomeningeal disease

          -  Chemotherapy within 21 days of first G1T38 dose

          -  Investigational drug within 28 days of first G1T38 dose

          -  Concurrent radiotherapy, radiotherapy within 14 days of first G1T38 dose, previous
             radiotherapy to the target lesion sites, or prior radiotherapy to > 25% of bone marrow

          -  Prior hematopoietic stem cell or bone marrow transplantation