Inclusion Criteria:

          -  Patient is ≥ 18 years-old at the time of PICF signature

          -  Patient is female with known menopausal status at the time of randomization or
             initiation of adjuvant ET (whichever occurs earlier), or male.

          -  Patient with histologically confirmed unilateral primary invasive adenocarcinoma of
             the breast with a date of initial cytologic or histologic diagnosis within 18 months
             prior to randomization.

          -  Patient has breast cancer that is positive for ER and/or PgR

          -  Patient has HER2-negative breast cancer

          -  Patient has available archival tumor tissue from the surgical specimen

          -  Patient after surgical resection where tumor was removed completely, with the final
             surgical specimen microscopic margins free from tumor, and belongs to one of the
             following categories: anatomic stage group II or III

          -  If indicated, patient has completed adjuvant and/or neoadjuvant chemotherapy according
             to the institutional guidelines

          -  If indicated, patient has completed adjuvant radiotherapy according to the
             institutional guidelines

          -  Patient has an Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1.

          -  Patient has no contraindication for the adjuvant ET in the trial and is planned to be
             treated with ET for 5 years

        Exclusion Criteria:

          -  Patient has received any CDK4/6 inhibitor

          -  Patient has received prior treatment with tamoxifen, raloxifene or AIs for reduction
             in risk ("chemoprevention") of breast cancer and/or treatment for osteoporosis within
             the last 2 years prior to randomization. Patient is concurrently using hormone
             replacement therapy.

          -  Patient has received prior treatment with anthracyclines at cumulative doses of 450
             mg/m² or more for doxorubicin, or 900 mg/m² or more for epirubicin.

          -  Patient with a known hypersensitivity to any of the excipients of ribociclib and/or ET

          -  Patient with distant metastases of breast cancer beyond regional lymph nodes (stage IV
             according to AJCC 8th edition) and/or evidence of recurrence after curative surgery.

          -  Patient is concurrently using other anti-neoplastic therapy with the exception of
             adjuvant ET

          -  Patient has had major surgery, chemotherapy or radiotherapy within 14 days prior to
             randomization

          -  Patient has not recovered from clinical and laboratory acute toxicities related to
             prior anti-cancer therapies

          -  Patient has a concurrent invasive malignancy or a prior invasive malignancy whose
             treatment was completed within 2 years before randomization

          -  Patient has known HIV infection, Hepatitis B or C infection

          -  Clinically significant, uncontrolled heart disease and/or cardiac repolarization
             abnormality

          -  Patient is currently receiving any of the following substances within 7 days before
             randomization - Concomitant medications, herbal supplements, and/or fruits that are
             known as strong inhibitors or inducers of CYP3A4/5 or Medications that have a narrow
             therapeutic window and are predominantly metabolized through CYP3A4/5

          -  is currently receiving or has received systemic corticosteroids ≤ 2 weeks prior to
             starting trial treatment

          -  Patient has impairment of GI function or GI disease that may significantly alter the
             absorption of the oral trial treatments

          -  Patient has any other concurrent severe and/or uncontrolled medical condition that
             would, in the Investigator's judgment, cause unacceptable safety risks, contraindicate
             patient participation in the clinical trial or compromise compliance with the protocol

          -  Participation in other studies involving investigational drug(s) within 30 days prior
             to randomization or within 5 half-lives of the investigational drug(s) (whichever is
             longer), or participation in any other type of medical research judged not to be
             scientifically or medically compatible with this trial.

          -  Pregnant or breast-feeding (lactating) women or women who plan to become pregnant or
             breast-feed during the trial