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