A Prospective, Randomized Multicenter, Open-label Comparison of Preoperative Trastuzumab Emtansine (T-DM1) With or Without Standard Endocrine Therapy vs. Trastuzumab With Standard Endocrine Therapy Given for Twelve Weeks in Patients With Operable HER2+/HR+ Breast Cancer Within the ADAPT Protocol.

Inclusion Criteria:

* Female patients, age at diagnosis 18 years and above (consider patients at 70 years and above for ADAPT Elderly)
* Histologically confirmed unilateral primary invasive carcinoma of the breast
* Clinical T1 - T4 (except inflammatory breast cancer)
* All clinical N (cN)
* No clinical evidence for distant metastasis (M0)
* Known HR status and HER2 status (local pathology) Tumor block available for central pathology review
* Performance Status ECOG ≤ 1 or KI ≥ 80%
* Negative pregnancy test (urine or serum) within 7 days prior to start of induction treatment in premenopausal patients
* Written informed consent prior to beginning specific protocol procedures, including expected cooperation of the patients for the treatment and follow-up, must be obtained and documented according to the local regulatory requirements
* The patient must be accessible for treatment and follow-up

Additional Inclusion criteria for participation in the HER2+/HR+ sub-protocol:

* Confirmed ER and/or PR positive and HER2+ by central pathology
* Clinical cT1c - T4a-c (participation of patients with tumors \>cT2 is strongly recommended)
* All clinical N (participation of patients with cN0, if cT1c is strongly recommended)
* Patients must qualify for neoadjuvant treatment
* LVEF \> 50%; LVEF within normal limits of each institution measured by echocardiography and normal ECG (within 42 days prior to induction treatment)

Exclusion Criteria:

* Known hypersensitivity reaction to the compounds or incorporated substances
* Prior malignancy with a disease-free survival of \< 10 years, except curatively treated basalioma of the skin, pTis of the cervix uteri
* Non-operable breast cancer including inflammatory breast cancer
* Previous or concurrent treatment with cytotoxic agents for any reason after consultation with the sponsor
* Concurrent treatment with other experimental drugs. Participation in another clinical trial with any investigational not marketed drug within 30 days prior to study entry
* Male breast cancer
* Concurrent pregnancy; patients of childbearing potential must implement
* a highly effective (less than 1% failure rate) non-hormonal contraceptive measures during the study treatment
* Breast feeding woman
* Sequential breast cancer
* Reasons indicating risk of poor compliance Patient not able to consent

Additional Exclusion Criteria for participation in the HER2+/HR+ sub-protocol:

* Known polyneuropathy ≥ grade 2
* Severe and relevant co-morbidity that would interact with the application of cytotoxic agents or the participation in the study
* Inadequate organ function (e.g. hepatic impairment, pulmonary disease, etc.)
* Uncompensated cardiac function (current unstable ventricular arrhythmia
* requiring treatment, history of symptomatic CHF NYHA classes II-IV), history of myocardial infarction or unstable angina pectoris within 6 months of enrollment, history of severe hypertension, CAD - coronary artery disease)
* Severe dyspnea
* Pneumonitis

Abnormal blood values:

* Thrombocytopenia \> CTCAE grade 1
* Increases in ALT/AST \> CTCAE grade 1
* Hypokalaemia \> CTCAE grade 1
* Neutropenia \> CTCAE grade 1
* Anaemia \> CTCAE grade 1