Cathepsin D is a protein that in humans is encoded by the CTSD gene. The main function of cathepsin D is to degrade proteins and activate precursors of bioactive proteins in pre-lysosomal compartments. This proteinase, which is a member of the peptidase A1 family, has a specificity similar to but narrower than that of pepsin A. Transcription of the CTSD gene is initiated from several sites, including one that is a start site for an estrogen-regulated transcript. Mutations in this gene are involved in the pathogenesis of several diseases, including breast cancer and possibly Alzheimer disease. Homozygous deletion of the CTSD gene leads to early lethality in the postnatal phase. Over-expression of cathepsin D stimulates tumorigenicity and metastasis as well as initiation of tumor apoptosis. This protease has been regarded an independent marker of poor prognosis in breast cancer being correlated with the incidence of clinical metastasis. Knock-out of CTSD gene would cause intestinal necrosis and hemorrhage and increase apoptosis in thymus, indicating that cathepsin D is required in certain epithelial cells for tissue remodeling and renewal.