Cathepsins are proteases that are distinguished by their structure, catalytic mechanism, and the proteins they cleave. Cathepsin D is a lysosomal aspartyl protease that is expressed in all cells. In certain tumor cells, Cathepsin D is abnormally processed and it is suggested that cathepsin D plays an important role in malignant transformation. It may be a useful prognostic indicator for breast cancer and possibly Alzheimer's disease.