Cathepsin E (CTSE) is a gastric aspartyl protease that functions as a disulfide-linked homodimer. It is a member of the Peptidase C1 family, and has a specificity similar to that of Pepsin A and Cathepsin D. CTSE is localized to the endoplasmic reticulum and Golgi apparatus, while the mature enzyme is localized to the endosome. It is expressed abundantly in the stomach, the Clara cells of the lung and activated B-lymphocytes, and at lower levels in lymph nodes, skin and spleen. CTSE is an intracellular proteinase that have a role in immune function, activation-induced lymphocyte depletion in the thymus, neuronal degeneration and glial cell activation in the brain. Futhermore, it probably involved in the processing of antigenic peptides during MHC class II-mediated antigen presentation.