In all type of solid tumors it is possible to distinguish two different compartments, the parenchyma and the stroma. While tumor parenchyma consists of the tumor cells themselves, tumor stroma comprises a heterogeneous population of cells including fibroblast, adypocites, endothelial and immune cells. Although cancer drug development traditionally was focused on targeting tumor cells, emphasis has recently shifted toward the tumor microenvironment, for novel therapeutic and prevention strategies. While standard therapies are often quite effective for the majority of breast cancer patients, acquired resistance and subsequent recurrence remain significant clinical problems. The aim of this project is to study the contribution of the stroma to tumor regression after endocrine therapy in an experimental model of breast cancer, as well as in human breast tumors collected from clinical biopsy material. To our knowledge there are no reports on the role of tumor stroma in the post-therapeutic regression response. Understanding how stromal influences affect clinical response could provide the basis for the development of more effective and selective antitumor therapies or potentiate therapies that already exist.