Inclusion Criteria: 1. All patients, independent from gender 2. Patient must be ≥18 years at diagnosis 3. The patient must be capable of giving informed consent and be willing and able to comply with the requirements and restrictions in this protocol and accessible for treatment and follow-up 4. Sign informed consent prior to any study-specific procedures. 5. Histologically confirmed unilateral, primary invasive carcinoma of the breast Note: bilateral, multicentric, or multifocal carcinoma may only be included after consultation of Sponsor. 6. Histologically confirmed diagnosis of primary hormone-receptor-positive (HR+) (i.e., oestrogen-receptor (ER) ≥ 10% and progesterone-receptor PR ≥ 10%) early breast cancer by local laboratory Note: ER positive according to ASCO / AGO Guidelines, ER 1-10% (low) is not defined as HR+. 7. Patient has HER2-negative breast cancer defined as a negative in-situ hybridization test or an IHC status of 0, 1+, or 2+, if IHC is 2+, a negative in-situ hybridization (FISH, CISH, or SISH) test is required (based on the most recently analysed tissue sample and all tested by a local laboratory). 8. No evidence of distant metastasis (confirmed by CT thorax / abdomen, X-ray chest, ultrasound liver, bone scan, or PET-CT, respectively, performed within clinical routine). 9. High genomic risk assessment within clinical routine (Oncotype DX® preferred; In those cases, where Oncotype Dx® is not possible in clinical routine, Oncotype Dx® should be assessed retrospectively after inclusion of the patient and as a study specific measure, provided sufficient tumour tissue from primary diagnosis is available.) 10. 10\. Completed 2-6 weeks of endocrine induction treatment and Ki-67 response assessment Note: 2-4 weeks recommended, up to 6 weeks allowed. Endocrine induction is highly recommended, but if endocrine induction therapy could not be performed or ET response is not representative, clinical factors should be used. 11. 11\. Completed (neo)adjuvant chemotherapy, if applicable 12. Completed radiotherapy, if applicable 13. Patient meets any of the following three conditions at end of primary treatment (including endocrine induction treatment, biopsy/surgery, and if necessary, chemotherapy and radiotherapy and up to 12 months standard-of-care endocrine treatment, excluding previous treatment \> 4 weeks with any SERD): Pathological Stage \* Genomical High-Risk (Oncotype Dx®)\*\* Age Clinical High-Risk Factors Stage I T1 N0 RS\>25 Any age High risk (≤ 1 factor applies): * ET non-response (post ET Ki-67 \>10%)\*\*\* * No Chemotherapy * G3 or PR negative and Ki-67 \>25%\*\*\* * Non-pCR after NACT\* RS 16-25 Age \<50 High risk (≤ 1 factor applies): * ET non-response (post ET Ki-67 \>10%)\*\*\* * No Chemotherapy * G3 or PR negative and Ki-67 \>25%\*\*\* * Non-pCR after NACT\* Any genomic risk Any age G3 and Ki-67\>40% Stage IIa with T2 N0 RS 0-25 Age\>50 High risk (≤ 1 factor applies): * ET non-response (post ET Ki-67 \>10%)\*\*\* * G3 and Ki-67\>40% * G3 or PR negative and Ki-67 \>25%\*\*\* * Non-pCR after NACT\* RS 0-15 Age\<50 No chemotherapy AND high risk (≤ 1 factor applies): * ET non-response (post ET Ki-67 \>10%)\*\*\* * G3 and Ki-67\>40% * G3 or PR negative and Ki-67 \>25%\*\*\* * Non-pCR after NACT\* RS 16-25 Age\<50 No chemotherapy, AND/OR high risk (≤ 1 factor applies): * ET non-response (post ET Ki-67 \>10%)\*\*\* * G3 and Ki-67\>40% * G3 or PR negative and Ki-67 \>25%\*\*\* * Non-pCR after NACT\* RS \>25 Any age Any clinical risk Any genomic risk Any age G3 and Ki-67 \>40% Stage IIa with T1 N1, G1-2 RS 0-25 Age\>50 High risk (≤ 1 factor applies): * ET non-response (post ET Ki-67 \>10%)\*\*\* * PR negative and Ki-67 \>25%\*\*\* * Non-pCR after NACT\* * 3 positive LN RS 0-25 Age \<50 No chemotherapy, AND/OR high risk (≤ 1 factor applies): * ET non-response (post ET Ki-67 \>10%)\*\*\* * PR negative and Ki-67 \>25%\*\*\* * Non-pCR after NACT\* * 3 positive LN RS\>25 Any age Any clinical risk Stage IIb with T3 N0 or T2 N1, G1-2 Any genomic risk Any age Any clinical risk \* In patients treated by neoadjuvant chemotherapy, clinical stage should be used for inclusion \*\* In stage I-IIa and N0 patients with unknown genomic risk prior to inclusion, Oncotype Dx® Test should be performed on untreated tumour tissue within clinical routine. Results of other genomic tests, if already performed within the clinical routine, may be considered. In such cases, inclusion is only possible if clinical high-risk criteria apply and after consultation with sponsor. In those cases, where Oncotype Dx® is missing from clinical routine and in the N1-situation, Oncotype Dx® should be assessed retrospectively after inclusion of the patient and as a study specific measure, provided sufficient tumour tissue from primary diagnosis is available. \*\*\* Use of clinical factors is recommended in patients with unknown or not representative ET response. 14. No contraindication for adjuvant SoC endocrine treatment 15. No contraindication for elacestrant treatment 16. No contraindication for ribociclib treatment, if medically indicated, and adequate washout time for CYP3A4 inducers/inhibitors and QT time-prolonging drugs is given 17. Tumour block for central pathology review (core biopsy of initial diagnosis and biopsy/surgery sample of definite surgery), if available 18. Performance Status ECOG ≤ 1 or Karnofsky Index ≥ 80% 19. Laboratory requirements (female and male patients, not older than 14 days prior to date of informed consent) * absolute neutrophil count ≥ 1.5 × 109/L, * platelets ≥ 100 × 109/L, * haemoglobin ≥ 9.0 g/dL, * INR ≤ 1.5, * serum creatinine \< 1.5 × ULN OR clearance ≥30 mL/min for participant with creatinine levels \>1.5 × institutional ULN * total bilirubin \< ULN, except for patients with Gilbert's Syndrome who may only be included if the total bilirubin is ≤ 3.0 × ULN or direct bilirubin ≤ 1.5 × ULN, * aspartate transaminase (AST) \< 2.5 × ULN, * alanine transaminase (ALT) \< 2.5 × ULN, * Screening lipid panel fasting levels: total cholesterol ≤400 mg/dL AND/OR triglycerides \<500 mg/dL. Note: Patients with lipid panel fasting levels NOT meeting the above criteria may consider initiating therapy for lipid management per local guidelines and will be allowed to be included once the lipid levels meet the inclusion criteria. 20. Clinical assessments: • normal electrocardiogram within 6 weeks prior to randomization (QTcF interval at screening \<450msec using Fridericia's correction, mean resting heart rate 50-90 bpm) 21. Ability to swallow tablets 22. Contraception A. Female patients of childbearing potential at inclusion must have a negative pregnancy test (serum) and additionally fulfil either one of the following conditions: * surgically sterile, * or carry a non-hormone releasing intrauterine device (combined with a barrier method), * or having received tubal ligation/occlusion (combined with a barrier method), * or using a highly effective contraceptive method from the time they sign consent, during participation in the study until end of study, at least 4 months (120 days) after the last dose of ELA, and for at least 21 days after the last dose of RIBO) to prevent pregnancy; ova donation or preservation is also not allowed within this time frame * Total/true abstinence: When the patient refrains from any form of sexual intercourse and this is in line with their usual and/or preferred lifestyle; this must continue from the time they sign consent, during participation in the study until end of study, at least 4 months (120 days) after the last dose of ELA, and for at least 21 days after the last dose of RIBO) to prevent pregnancy; * Vasectomised sexual partner (with participant assurance that partner received post-vasectomy confirmation of azoospermia) combined with a barrier method or sexual partner with bilateral orchiectomy. * Hormonal contraception is not acceptable. B. Male patients must either be * surgically sterile * or using a highly effective method of contraception from the time they sign consent, during participation in the study until end of study, at least 4 months (120 days) after the last dose of ELA, and for at least 21 days after the last dose of RIBO) to prevent pregnancy in a partner; sperm donation or preservation is also not allowed within this time frame * Male patients who intend to be sexually active with a woman of childbearing potential, must use a condom plus spermicide from the time they sign consent, during participation in the study until end of study, at least 4 months (120 days) after the last dose of ELA, and for at least 21 days after the last dose of RIBO) to prevent pregnancy in a partner; * Highly effective methods of contraception should be considered in female partners of men taking elacestrant and/or ribociclib who are of childbearing potential. Exclusion Criteria: 1. Known hypersensitivity to any of the compounds or incorporated substances of the IMPs 2. Prior malignancy with a disease-free survival of \<5 years, except curatively treated basalioma of the skin or pTis of the cervix uteri 3. Any history of invasive cancer within the last 10 years Note: adequately treated, basal or squamous-cell skin carcinoma, non-melanomatous skin cancer, curatively resected cervical cancer, and contralateral DCIS treated by mastectomy (contralateral in relation to current invasive breast cancer diagnosis) are excepted. Previous ipsilateral DCIS, irrespective of treatment, is excluded! 4. Patient with distant metastases of breast cancer beyond regional lymph nodes. 5. Concurrent treatment with cytotoxic agents for any non-oncological reason unless clarified with sponsor 6. Concurrent treatment with other experimental drugs 7. Participation in another interventional clinical trial with or without any investigational, not marketed drug within 30 days or 5 half-lives of the respective drug, whichever is longer, prior to study entry. In case of other interventional trial contact Sponsor. 8. Previous treatment (\>4 weeks) with any SERD 9. Concurrent pregnancy; patients of childbearing potential or potentially childbearing partners of male patients must implement a highly effective (less than 1% failure rate) non-hormonal contraceptive measures during the study treatment 10. Breast feeding woman 11. Use of oral, transdermal, injected, or implanted hormonal methods of contraception as well as hormonal replacement therapy (oestrogen or progesterone). 12. Reasons indicating risk of poor compliance 13. Patient not able to consent 14. Patient has not recovered from clinical and laboratory acute toxicities related to prior anticancer therapies to NCI CTCAE version 5.0 Grade ≤ 1. 15. Severe and relevant co-morbidity that would interact with the application of endocrine treatment of any kind or the participation in the study 16. For patients planned for ribociclib treatment: Clinically significant, uncontrolled heart disease and/or cardiac repolarization abnormality, including any of the following: * history of myocardial infarction (MI), angina pectoris, symptomatic pericarditis, or coronary artery bypass graft (CABG) within 6 months prior to study entry, * documented cardiomyopathy, * left ventricular ejection fraction (LVEF) \< 50 % as determined by multiple gated acquisition (MUGA) scan or echocardiogram (ECHO), * long QT syndrome, family history of idiopathic sudden death, congenital long QT syndrome, or any of the following: * risk factors for Torsades de Pointe (TdP, polymorphic ventricular tachycardia in patients with long QT syndrome) including uncorrected hypokalaemia or hypomagnesemia, history of cardiac failure, or history of clinically significant/ symptomatic bradycardia, * concomitant medications with a known risk to prolong the QT interval and/or known to cause Torsades de Pointe that cannot be discontinued or replaced by safe alternative medication (e.g., within 5 half-lives or 7 days prior to starting study drug), * inability to determine the QTcF interval, * clinically significant cardiac arrhythmias (e.g., ventricular tachycardia), complete left-bundle branch block, high-grade AV block (e.g., bi-fascicular block, Mobitz type II, and 3rd-degree AV block), * systolic blood pressure (SBP) \> 160 or \< 90 mmHg. 17. Patient has impairment of gastrointestinal (GI) function or GI disease that may significantly alter the absorption of the study drugs (e.g., uncontrolled ulcerative diseases, uncontrolled nausea, vomiting, diarrhoea, malabsorption syndrome, or small-bowel resection). 18. Uncontrolled infection requiring i.v. antibiotics, antivirals, or antifungals 19. Active primary immunodeficiency, known human immunodeficiency virus (HIV) infection. Patients should be tested for HIV prior to randomization if required by local regulations or ethics committee (EC). Patients who test positive for HIV-antibody are excluded. 20. Patient has known active hepatitis-B-virus (HBV) or hepatitis-C-virus (HCV) infection. Screening for HBV or HBC-infection and testing for hepatitis-B or -C is highly recommended according to current valid (local) clinical guidelines, but neither part of the interventional study procedures, nor required for enrolment. 21. Patient has received live vaccines within 30 days prior to randomization. 22. Patient was submitted to an institution by virtue of an order of a court or a governmental authority must be excluded from participation.