Understanding and preventing the development of distant metastases is the most important aim in research and treatment of malignant tumors, including breast cancer. In patients with primary breast cancer without lymph node metastases who are under 50 years of age, approximately 25% will develop distant metastases after 5 years. When treated with adjuvant chemotherapy, this can be reduced to approximately 18%. When lymph node metastases are present at primary treatment, approximately 50% of the patients will develop distant metastases and this figure can be reduced to less than 40% by adjuvant chemotherapy treatment. In elderly women (50-69 years) the benefit of chemotherapy decreases from approximately 10% absolute benefit to 5% absolute benefit [1]. These numbers illustrate on the one hand the benefit for adjuvant chemotherapy, on the other hand that a large number of patients will also remain free of recurrence without adjuvant chemotherapy and suffer from the site effects without any benefit from the toxic treatment. It will be of great clinical benefit to be able to better predict which tumors will develop distant metastases, as adjuvant systemic treatment can than be better tailored to individual patients. In addition, identification of such predictive factors for distant metastases will lead to more insight in the biological processes leading to the development of distant metastases.