Inclusion criteria:

          -  Signed, written informed consent in study participation

          -  The primary tumor and/or biopsies from metastatic sites or locoregional recurrences
             have been confirmed as HER2-positive (FISH-positive or IHC 3+) and hormone receptor
             positive breast cancer by histopathology according to local testing

          -  Metastatic breast cancer or locally advanced BC, which cannot be treated by surgery or
             radiotherapy only

          -  Pre- and postmenopausal women are allowed

          -  No more than two prior chemotherapies for metastatic disease

          -  No more than two prior anti-HER2 therapies for metastatic disease

          -  Pertuzumab retreatment is allowed if prior pertuzumab treatment was finished 12 months
             before

          -  At least one measurable lesion assessable using standard techniques by Response
             Evaluation Criteria In Solid Tumors Version 1.1 (RECIST v1.1)

          -  Tumor evaluation according to RECIST version 1.1 has been performed within 4 weeks
             before randomization based on local assessment

          -  Age ≥ 18 years

          -  Standard 12-lead ECG values assessed by the local laboratory:

               -  QTcF interval at screening < 450 msec (using Fridericia's correction)

               -  Resting heart rate 50-90 bpm

          -  Left ventricular cardiac ejection fraction (LVEF) ≥ 50% at baseline (as measured by
             echocardiogram)

          -  ECOG Score ≤ 2

          -  Adequate organ function within 14 days before randomization, evidenced by the
             following laboratory results below:

               -  absolute neutrophil count ≥ 1500 cells/µL,

               -  platelet count ≥ 100000 cells/µL,

               -  hemoglobin ≥ 9 g/dL,

               -  ALT (SGPT) ≤ 2.0 × ULN (≤ 3.0 × ULN in case of liver metastases)

               -  AST (SGOT) ≤ 2.0 × ULN (≤ 3.0 × ULN in case of liver metastases)

               -  bilirubin ≤ 1.5 × ULN (with the exception of Gilbert's syndrome)

               -  creatinine ≤ 2.0 mg/dl or 177µmol/L INR ≤ 1,5

          -  Patients must have the following laboratory values within normal limits or corrected
             to within normal limits with supplemets before the first dose of study medication:

               -  Sodium

               -  Potassium

               -  Total calcium

          -  In case of patients of child bearing potential:

        Negative serum pregnancy test at baseline (within 7 days prior to randomization) and
        agreement to remain abstinent (if it is in line with the preferred and usual lifestyle) or
        use single or combined non-hormonal contraceptive methods that result in a failure rate of
        < 1% per year during the treatment period and for at least 7 months after the last dose of
        study treatment

        Exclusion criteria:

        Patients will be excluded from the study for any of the following reasons:

          -  History of hypersensitivity reactions attributed to trastuzumab, pertuzumab,
             ribociclib or to other components of drug formulation

          -  Mandatory need for cytostatic treatment at time of study entry based on clinical
             judgment and national/international treatment guidelines

          -  Known CNS metastases

          -  Any concurrent severe, uncontrolled systemic disease, social or psychiatric condition
             that might interfere with the planned treatment and with the patient's adherence to
             the protocol

          -  Progression on prior Pertuzumab therapy

          -  Treatment with Pertuzumab within the last 12 months

          -  Prior treatment with any mTOR- or CDK4/6-inhibitor

          -  Treatment with any other investigational agents during trial

          -  Known hypersensitivity to lecithin (soya) or peanuts

          -  Life expectancy < 6 months

          -  Patients with pre-existing grade ≥2 peripheral neuropathy are excluded from
             taxane-based chemotherapy

          -  History of serious cardiac disease, including but not confined to:

               -  history of documented heart failure or systolic dysfunction (LVEF < 50%)

               -  high-risk uncontrolled arrhythmias i.e., atrial tachycardia with a heart rate
                  ≥100/min at rest, significant ventricular arrhythmia (ventricular tachycardia) or
                  higher-grade AV-block (second degree AV-block Type 2 [Mobitz 2] or third degree
                  AV-block)

               -  angina pectoris requiring anti-anginal medication

               -  clinically significant valvular heart disease

               -  evidence of transmural infarction on ECG

               -  poorly controlled hypertension (e.g., systolic >180 mm Hg or diastolic >100 mm
                  Hg)

               -  any other cardiac condition, which in the opinion of the treating physician would
                  make this protocol unreasonably hazardous for the patient

          -  Dyspnea at rest or other diseases that require continuous oxygen therapy

          -  Patients with poorly controlled diabetes or with evidence of clinically significant
             diabetic vascular complications

          -  Patients with known infection with HIV, hepatitis B virus, or hepatitis C virus

          -  Male patients

          -  Pregnant, lactating or women of childbearing potential without a negative pregnancy
             test (serum) within 7 days prior to randomization, irrespective of the method of
             contraception used

          -  Medical or psychological conditions that would not permit the subject to complete the
             study or sign informed consent

          -  Participation in another clinical study within the 30 days before registration

          -  Legal incapacity or limited legal capacity