PROOFS-Registry - Premenopausal Women With Breast Cancer Optimally Treated With OFS

Inclusion Criteria: Patients are eligible for participation in the registry only if they meet all the following criteria: * Female breast cancer patients * Pre- or perimenopausal at registry entry (age \<60 years and state after hysterectomy or amenorrhea for \<12 months; confirmation by blood hormone levels (FSH and estradiol in premenopausal range as per local normal range) recommended) * Primary tumor diagnosis not older than three months prior to inclusion (primary diagnosis defined as date of initial tumor biopsy) * Estrogen- and/or progesterone-receptor-positive/HER2 negative early breast cancer without any clinical signs of metastases * Adequate risk for recurrence: * intermediate clinical risk for recurrence, defined as (clinical in case of neoadjuvant treatment): * c/pT1 and * c/pN0 and * Ki-67 15-24% or * G2 or * patients, who do not meet these criteria but are at intermediate clinical risk for recurrence at investigator decision (e.g., very young age, low expression of hormone receptors, existing co-morbidities, familial cancer burden, etc.) can be included on individual decision basis or * high clinical risk for recurrence, defined as either (clinical in case of neoadjuvant treatment): * c/pT2-4 or * c/pN1 or * Ki-67 ≥25% or * G3 * Low genomic risk of recurrence by MammaPrint® (tested on treatment naïve tumor specimen) * Luminal-type by BluePrint® * Treatment according to standard-of-care (e.g., AGO Guidelines) planned or started (until completion of local therapy the latest (including started or completed endocrine induction therapy), started, or planned adjuvant or neoadjuvant treatment) * Availability of untreated tumor material (core biopsy if preoperative endocrine therapy performed or neoadjuvant treatment intended or surgery specimen) * Capability to give written informed consent * Nodal positive patients will be accepted to the registry up to 25% of the genomic low/ultralow-risk population (n=441). Exclusion Criteria: Patients will not be eligible for the registry for any of the following reasons: * Any other genomic testing, besides MammaPrint®, has been performed on the tumor material * Medical or psychological conditions that would not permit the patient to sign informed consent * Legal incapacity or limited legal capacity * Current participation in any interventional clinical trial which tests anticancer drugs, immunotherapeutics, or antibody treatment for any type of neoplasm * Non-compliance of the patient