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