Solid human tumors and their surrounding microenvironment are hypothesized to coevolve in a manner that promotes tumor growth, invasiveness, and spread. Mouse models of cancer have focused on genetic changes in the epithelial tumor cells and therefore have not robustly tested this hypothesis. We have recently developed a murine breast cancer model that ablates the PTEN tumor suppressor pathway in stromal fibroblasts. Remarkably, the model resembles human breast tumors both at morphologic and molecular levels. We propose that such models reflect subtypes of tumor - stromal coevolution relevant to human breast cancer, and will therefore be useful in defining the mechanisms that underpin tumor - stroma cross-talk. Additionally, these models should also aid in molecularly classifying human breast tumors on the basis of both the microenvironment subtypes they contain as well as on the tumor subtype.