Why primary tumors metastasize preferentially to particular organs is an important but still unanswered question in cancer biology. The tumor presumably communicates with the target organ, but how this long-distance molecular conversation occurs has been difficult to envisage. Enter exosomes, mysterious lipid vesicles that have been turning up in many diverse areas of biomedical research. Costa-Silva et al. show that well in advance of metastasis, primary tumor cells secrete exosomes that carry a specific molecular cargo to the target organ. This cargo helps transform the organ into a hospitable niche that supports the growth of metastatic cells. In the case of mouse pancreatic cancer, the exosomes carried a protein that induced a proinflammatory, tumor cell–friendly milieu in the liver….
