Inclusion Criteria:

          -  For Part 1, postmenopausal women only

          -  For Parts 2 and 3, any menopausal status

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

          -  For Part 1, prior treatment with less than 4 prior lines of chemotherapy

          -  For Part 2, prior treatment with less than 2 prior line of chemotherapy

          -  For Part 3, prior treatment with no more than 1 prior line of chemotherapy

          -  For Parts 1 and 2, prior treatment with less than 4 prior endocrine therapies for
             metastatic breast cancer

          -  For Part 3, prior treatment with no more than 1 prior line of endocrine therapies for
             metastatic breast cancer

          -  For Parts 1 and 2, 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

               -  Progressed after the end of prior aromatase inhibitor therapy for
                  advanced/metastatic breast cancer

          -  For Part 3, patients must satisfy 1 of the following criteria for prior therapy:

               -  Received ≥ 24 months of endocrine therapy in the adjuvant setting prior to
                  recurrence or progression

               -  Received ≥ 6 months of endocrine therapy in the advanced/metastatic setting prior
                  to progression

          -  For Part 1, evaluable or measurable disease

          -  For Parts 2 and 3, evaluable (approximately 25%) or measurable disease (approximately
             75%) as defined by RECIST, Version 1.1 including bone-only disease

          -  ECOG performance status 0 to 1

          -  Adequate organ function

        Exclusion Criteria:

          -  For Part 3, prior treatment with CDK4/6 inhibitor, investigational oral SERDs or
             SERCAs in any setting

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

          -  Anticancer therapy within 14 days of first G1T48 dose or within 28 days for
             antibody-based therapy

          -  Concurrent radiotherapy, radiotherapy within 14 days of first G1T48 dose, previous
             radiotherapy to the target lesion sites, or prior radiotherapy to > 25% of bone marrow

          -  Prior hematopoietic stem cell or bone marrow transplantation