Malignant tumor cells from a primary tumor have to pass into the bloodstream in order to form metastases in other organs. It is accepted in medical research that, in breast cancer for example, tumor cells first of all enter the vascular system and then colonize the sentinel lymph nodes closest to the primary tumor. From there they travel along the lymph channels via further secondary lymph nodes and eventually find their way into the bloodstream. In a mouse model, scientists have now discovered another highly efficient and much faster route via which tumor cells already in the sentinel lymph nodes pass through their blood vessels to "invade" the vascular system, causing pulmonary metastases.