Cathepsin D is a protein encoded by the CTSD gene. Cathepsin D is a lysosomal aspartate protease that is activated at low pH. It is widely present in vertebrates, fungi, retroviruses, and some plant viruses. Usually, cathepsin D first synthesizes non catalytic precursor enzymes in the rough endoplasmic reticulum, removes signal peptides, enters the Golgi apparatus for protein cleavage and processing, and finally enters the lysosome through the mannose-6-phosphate pathway. In mammals, the active mature protein form contains 1 light chain and 1 heavy chain. Cathepsin D is widely expressed and participates in protein hydrolysis degradation, cell invasion, and apoptosis. The mutation of cathepsin D gene is related to the pathogenesis of several diseases, including breast cancer and possible Alzheimer’s disease, and has been considered as a tumor marker of breast cancer.