The expression and functionality of MSF during the progression of breast cancer. MSF is concurrently expressed by a proportion of carcinoma cells, stromal fibroblasts and vascular endothelial cells (producer cells). In vitro and in vivo observations indicate that it affects a diverse range of phenotypic features of these same cells (now functioning as target cells). MSF activity may be mediated by autocrine mechanims (i.e. same producer and target cell population) and paracrine mechanisms (i.e. different producer and target cell populations). Both MSF expression and cellular response to it are matrix dependent. HA, hyaluronic acid.