Inclusion

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

          -  Patient is female with known menopausal status at the time of PICF signature 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 who 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 no contraindication for the adjuvant ET in the trial and is planned to be
             treated with ET for 5 years

        Exclusion

          -  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 PICF signature

          -  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 PICF signature

          -  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

          -  Patient participated in another interventional study and received treatment with an
             investigational product (or used an investigational device) within 30 days prior to
             randomization or within 5 half-lives of the investigational product, whichever is
             longer.

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