Ctss, as known as cathepsin S isoform 2, is a lysosomal cysteine protease of the papain family. This protein plays a major role in the processing of the MHC class II-associated invariant chain. It has been implicated in the pathogenensis of several diseases such as Alzheimer’s disease and degenerative disorders associated with the cells of the mononuclear phagocytic system. Also, it is less abundant in tissues than Cathepsins B, L and H. The highest levels of this protein have been found in lymph nodes, spleen and phagocytic cells. Recombinant mouse Ctss, fused to His-tag at C-terminus, was expressed in insect cell and purified by using conventional chromatography techniques.