Cancer cells are embedded in stroma, the connective tissue framework of solid tumors. It consists of nonmalignant hematopoietic and mesenchymal cells, as well as extracellular matrix. Whether stromal cells have an essential role in cancer development and growth has been long debated. On page 827 of this issue, Kraman et al. show that deleting a subpopulation of stromal fibroblasts arrests the growth of a solid tumor, an effect that depends on an immune response to the tumor. These results agree with other studies suggesting that immunizing against fibroblasts in tumors can unmask an immune response to cancer (, ).