Brain metastases can only develop if cancer cells first exit the fine blood vessels and enter into the brain tissue. To facilitate this step, cancer cells influence blood clotting, as Heidelberg scientists from the German Cancer Research Center and from Heidelberg University Hospital have now been able to show in mice. The cancer cells actively promote the formation of clots, which helps them to arrest in the brain capillaries and then penetrate through the vessel wall. Drugs that inhibit the clotting factor thrombin were able to reduce the number of brain metastases in this experimental model.