Schematic view of normal, in situ, invasive, and metastatic carcinoma progression. Normal breast ducts are composed of the basement membrane and a layer of luminal epithelial and myoepithelial cells. Cells composing the stroma include various leukocytes, fibroblasts, myofibroblasts, and endothelial cells. In in situ carcinomas the myoepithelial cells are epigenetically and phenotypically altered and their number decreases, potentially due to degradation of the basement membrane. At the same time, the number of stromal fibroblasts, myofibroblasts, lymphocytes, and endothelial cells increases. Loss of myoepithelial cells and basement membrane results in invasive carcinomas, in which tumor cells can invade surrounding tissues and can migrate to distant organs, eventually leading to metastases.