Inclusion Criteria:

          1. Personally signed written informed consent prior to beginning protocol specific
             procedures, including expected cooperation of the patient for the treatment and
             follow-up, must be obtained and documented according to the local regulatory
             requirements

          2. Women with proven diagnosis of advanced, defined as locally advanced inoperable or
             metastatic, adenocarcinoma of the breast

          3. Hormone-receptor-positive (HR+) disease, defined as estrogen-receptor-positive (ER+)
             and/or progesterone-receptor-positive (PgR+)

          4. Human epidermal growth factor receptor 2-negative (HER2-) disease (HER2 neg/+ or
             HER2++ with CISH/FISH neg.)

          5. Pre-/perimenopausal women receiving concomitant therapy with an luteinizing
             hormone-releasing hormone (LHRH) agonist / ovarian ablation or postmenopausal status

          6. Age ≥18 years

          7. Measurable disease as per Response Evaluation Criteria in Solid Tumors [RECIST] or
             bone-only disease

          8. Patients scheduled for palliative treatment with an combination partner for first- or
             later-line

          9. Eastern Cooperative Oncology Group (ECOG) performance status 0-2

         10. Adequate organ and marrow function

         11. Resolution of all acute toxic effects of prior therapy, including radiotherapy Grade
             <1 (except toxicities not considered a safety risk for the patient) and recovery from
             surgical procedures

         12. Fluent in spoken and written German

        Exclusion Criteria:

          1. Prior treatment with any CDK4/6 inhibitor

          2. Prior adjuvant therapy with the respective endocrine combination partner if last
             intake <12 months prior to entering the study

          3. Prior palliative therapy with the respective endocrine combination partner

          4. More than one prior palliative chemotherapy

          5. 5. Known hypersensitivity to letrozole, anastrozole, exemestane, fulvestrant or any of
             their excipients

          6. Current use of food or drugs known to be potent inhibitors or inducers of CYP3A4
             (refer to Appendix 15.4)

          7. Current use of preparations containing St. John's Wort

          8. Participation in other studies involving investigational drug(s) (Phases I-IV) within
             2 weeks before the current study treatment begins

          9. QTc > 480 msec on the screening ECG (using the QTcF formula and/or the QTcB (Bazett)
             formula); history of QT syndrome, Brugada syndrome or known history of QTc
             prolongation, or Torsade de Pointes

         10. High cardiovascular risk, including, but not limited to recent myocardial infarction,
             severe/unstable angina and severe cardiac dysrhythmias in the past 6 months prior to
             enrollment

         11. Patients with advanced symptomatic, visceral spread, that were at risk of
             life-threatening complications in the short term (including patients with massive
             uncontrolled effusions [pleural, pericardial, peritoneal], pulmonary lymphangitis, and
             over 50% liver involvement)

         12. Diagnosis of any second malignancy within the last 3 years prior to enrollment, except
             for adequately treated basal cell or squamous cell skin cancer, or carcinoma in situ
             of the cervix

         13. Known, not-irradiated CNS metastases