Cathepsin E (CTSE) is also known as Cd152,Ly56 and Ctla4,which can be cleaved into the following 2 chains: Cathepsin E form I and Cathepsin E form II. Cathepsin E belongs to the peptidase A1 family. CTSE is similar to cathepsin D, but slightly broader specificity. The proenzyme is localized to the endoplasmic reticulum and Golgi apparatus, while the mature enzyme is localized to the endosome. CTSE 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, but not expressed in resting B-lymphocytes. CTSE may have a role in immune function,activation-induced lymphocyte depletion in the thymus, and in neuronal degeneration and glial cell activation in the brain.At the same time, CTSE is also probably involved in the processing of antigenic peptides during MHC class II-mediated antigen presentation.