Cathepsin E is produced by superficial epithelial cells, whereas pepsinogen and progastricsin are produced by glandular epithelial cells. In addition, human and rat gastric cells and rat neutrophils gave a strong labeling for cathepsin E in the cistemae of the rough endoplasmic reticulum (ER) and the perinuclear envelope as well as labeling in the cytosol. Procathepsin E has unique structural characteristics compared to other aspartic proteinases. By electron immunochemistry, cathepsin E was localized to endosomal vesicles and ER of both intestinal and tonsillar M cells. The immunological protection or mucosal protection of the gut was also hypothesized as a physiologic function of cathepsin E. The gene of human cathepsin E was shown to contain nine exons with boundary regions which are very conserved in the family of aspartic proteinases. Cathepsin E is one of the least characterized human aspartic proteinases.