The factors associated with the progression of ductal carcinoma in situ (DCIS) to invasive breast cancer are poorly understood. Many studies of this subject focus on the role of molecular and genetic alterations in the neoplastic epithelial cells. However, emerging evidence suggests that transition from DCIS to invasive cancer is strongly dependent upon alterations in the microenvironment. The potential roles of myoepithelial cells and of stromal-epithelial interaction are of particular interest in this regard.