Hepsin is a membrane bound protease of 51 kDa present in mammalian cells. It contains a stretch of hydrophobic sequence of 27 amino acid residues in its N-terminal region. By employing fluorescent immunostaining of cells and western blot analysis of the various cell subfractions, the catalytic subunit (carboxyl terminal half) of hepsin is found at the cell surface. The hepsin gene is expressed in most tissues of a young adult baboon, particularly in the liver at high levels.