CTSD localizes to the lysosome and consists of a light chain and a heavy chain. CTSD is expressed in epithelial cells as well as in macrophages.CTSD is a lysosomal aspartyl protease that depends critically on protonation of its active site Asp residue and gets activated at pH 5 in endosome of hepatocytes. It has been suggested to facilitate cancer cell migration and invasion by digesting the basement membrane, extracellular matrix and xonnective tissue. In addition, CTSD has been used as a breast cancer tumor marker.