Pepsin is a major digestive enzyme produced in the stomach. The native enzyme is most active between 37 °C to 42 °C and at a pH range of 2.0 to 6.5. It uses a catalytic aspartate in the active site and preferentially cleaves connected hydrophobic and aromatic residues. Pepsin can be used in exposing antigens in formalin-fixed tissues to enhance binding in immunostaining and in in situ DNA hybridizations. It is also used in cleaving IgG antibodies to separate the F(ab’)2 fragments from the Fc regions.