Cathepsin D is an acidic lysosomal protease present in all cells. In estrogen receptor positive and negative breast cancer cell lines, the mRNA coding for pro-cathepsin D is overexpressed and sorting and maturation of the pro-enzyme are altered, leading to accumulation of the active proteinase in large endosomes and to secretion of the precursor (52K protein). In MCF7 cells, the cathepsin D mRNA is induced directly and transcriptionally by estrogens and indirectly by growth factors. In vitro, pro-cathepsin D is an autocrine mitogen on breast cancer cells and can be auto-activated to degrade extracellular matrix and proteoglycans and to activate other proteinases in acidic microenvironments. In patients, there is a significant correlation between high cathepsin D concentrations in the cytosol of primary breast cancer and development of metastasis. This marker is independent of other prognostic factors and appears to be particularly useful in lymph node-negative tumors. These results suggest that overexpression and possible derouting of cathepsin D plays an important role in invasion and metastasis of breast cancer.