Provides the silk fibroin thread with a sticky coating. Acts as a cement by sticking silk threads together. Produced exclusively in the middle (MSG) section of silk glands. Sericin is useful because of its properties. The protein resists oxidation, is antibacterial, UV resistant, and absorbs and releases moisture easily. Sericin can be cross-linked, copolymerized, and blended with other macromolecular materials, especially artificial polymers, to produce materials with improved properties. The protein is also used as an improving reagent or a coating material for natural and artificial fibers, fabrics, and articles. The materials modified with sericin and composites are useful as degradable biomaterials, biomedical materials, polymers for forming articles, functional membranes, fibers, and fabrics. The glue-like property of sericin is attributed to the hydrogen bonding between serine residues of sericin with serine residues in the fibroin structural components of silk fiber.