Cathepsin E is an intracellular aspartic protease of the pepsin family, which is structurally highly homologous to the lysosomal aspartic protease cathepsin D. Cathepsin E is less abundant than Cathepsin D and has a more limited distribution, which includes lymphoid tissues, gastrointestinal tract tissue, blood cells, microglia and activated B lymphocytes. It is not expressed in resting B lymphocytes.Cathepsin E may have a role in immune function. It is most likely involved in the processing of antigenic peptides during MHC class II-mediated antigen presentation. It may also play a role in activation-induced lymphocyte depletion in the thymus, and in neuronal degeneration and glial cell activation in the brain.