Ctsd, also known as cathepsin D, is peptidase belonging to the family of aspartic peptidases. It is intracellular catabolism in lysosomal compartments, other physiological effect include hormone and antigen processing. It plays an important role in ?-synuclein degradation, and neuronal survival. It has also been shown to mediate apoptosis induced by various stimuli and p53-dependent tumor suppression. Recombinant mouse Ctsd, fused to His-tag at C-terminus, was expressed in insect cell and purified by using conventional chromatography techniques.