Cathepsin D (CatD) is a ubiquitously expressed lysosomal aspartyl protease involved in the normal degradation of proteins apoptosis and autophagy. Human CatD is synthesized on the endoplasmic reticulum as a pre-pro-enzyme. Preprocathepsin D (412aa) is cleaved and glycosylated to form an inactive procathepsin D (392aa) and then further cleaved to generate an active intermediate (348aa) single-chain molecule. The active intermediate is further processed into mature two chain form of cathepsin D, this processing step is carried out by cathepsin B or L. The two-chain form consists of an amino terminal light chain and a carboxyl-terminal heavy chain. Additionally, several more amino acids are removed from the carboxyl terminus of the heavy chain. Procathepsin D (pCD), secreted from cancer cells, acts as a mitogen on both cancer and stromal cells and stimulates their proinvasive and pro-metastatic properties. Acid protease active in intracellular protein breakdown. Involved in the pathogenesis of several diseases such as breast cancer and possibly Alzheimer disease