PRIMe is a prospective, case-only trial designed to measure the impact of MammaPrint on physician chemotherapy intention in the two discordant groups (ET/POOR, CT/GOOD) in stage 1 and 2 HR-positive HER2-negative breast cancer patients. The design also provides for assessment of several important secondary indicators. Eligible patients will have their tumor sample analyzed for MammaPrint, BluePrint and TargetPrint. Patients cannot start treatment before the MammaPrint result is received and taken into consideration for the adjuvant treatment plan.

Inclusion Criteria:

• Women with histologically proven invasive stage 1 and 2 breast cancer
• Hormone receptor positive according to local standards
• HER2 negative - i.e. IHC 0-1+, or FISH or other ISH non-amplified (locally assessed)
• Axillary lymph node status: 0-3 involved (macro metastases i.e. >2mm OR micro metastases i.e. >0.2-2mm)
• ≥ 18 years of age at time of consent
• Patients must be eligible to receive adjuvant chemotherapy and endocrine therapy as defined by a good Karnofsky index, no hematological, cardiological or hepatic contraindications nor any impeding comorbidity
• Written informed consent

Exclusion Criteria:

• ≥4 involved axillary nodes
• Multi-centric disease with more than 2 clinically relevant lesions
• HR negative OR HER2 positive/amplified (locally assessed)
• Previous diagnosis of malignancy unless disease free for 10 years
• Metastatic disease
• Tumor sample shipped to Agendia with ≤ 30% tumor cells or that fails QA or QC criteria
• Women who have started or completed adjuvant chemotherapy or neo-adjuvant chemotherapy for current breast cancer

The study population will comprise patients for whom the investigator will advise either endocrine therapy or chemotherapy + endocrine therapy. Patients confirmed to be low/high risk by the gene signature will receive the respective treatment. Patients with discordant results will be investigated for investigator's chemotherapy intention after MammaPrint results are available.