CTSE is a gastric aspartyl protease that functions as a disulfide-linked homodimer. CTSE, which is a member of the peptidase C1 family, has a specificity similar to that of pepsin A and cathepsin D. It is an intracellular proteinase that does not appear to be involved in the digestion of dietary protein and is found in highest concentration in the surface of epithelial mucus-producing cells of the stomach. It is the first aspartic proteinase expressed in the fetal stomach and is found in more than half of gastric cancers. It appears, therefore, to be an oncofetal antigen. Transcript variants utilizing alternative polyadenylation signals and two transcript variants encoding different isoforms exist for this gene. Recombinant human CTSE protein, fused to His-tag at N-terminus, was expressed in E.coli.