Inclusion Criteria:

          1. Adult women (≥ 20 years of age)

          2. Diagnosis of adenocarcinoma of the breast with evidence of metastatic disease or
             advanced disease not amenable to resection or radiation therapy with curative intent.

          3. Documented evidence of HR+/HER2- tumor based on the patient's surgical specimen or
             most recent tumor biopsy.

          4. Initiating first or second line treatment at study entry with one of the following
             therapies:

             palbociclib plus endocrine therapy or endocrine monotherapy

          5. Eastern Cooperative Oncology Group (ECOG) performance status = 0~1.

          6. Owns or has regular access to an Apple iPhone or Android phone.

          7. Willing and able to complete collection of data via smartphone-based application.

          8. Willing and able to wear the wearable device for approximately 6 months.

          9. Evidence of a personally signed and dated informed consent document indicating that
             the patient has been informed of all pertinent aspects of the study.

         10. Able to read and understand Japanese

        Exclusion Criteria:

          1. The patient is participating in any interventional clinical trial that includes
             investigational or marketed products. Patients participating in other
             investigator-initiated research or non-interventional studies can be included as long
             as their standard of care is not altered by the study.

          2. The patient is on active treatment for other malignancies other than ABC.

          3. The patient's life style is fluctuating in weekly-basis (eg, shift-time worker), which
             may have high impact on physical activity assessment based to investigator's
             discretion