It consists mainly of two proteins, fibroin and sericin. There are many scleroprotein superfamilies including keratin, collagen, elastin, and fibroin. Silk III is a newly discovered structure of fibroin. The heavy fibroin protein consists of layers of antiparallel beta sheets. This impressive mechanical strength has made silk fibroin very competitive for applications in biomaterials. In addition, silk fibroin can be easily manipulated and sterilized. Silk fibroin contains hydrophobic beta sheet blocks, interrupted by small hydrophilic groups. I is the natural form of fibroin, as emitted from the "Bombyx mori" silk glands. Silk fibroin has been additionally found to support stem cell proliferation in vitro. SHG also reveals fibroin in silk, myosin in muscles and biosynthetized cellulose. In recent studies, silk fibroin has been found to posses antiagulation properties and platelet adhesion. The silk worm produces fibroin with three chains, the light, heavy, and the glycoprotein P25. Fibroin is known to arrange itself in three structures, called silk I, II, and III. Fibroin is made up of the amino acids Gly-Ser-Gly-Ala-Gly-Ala and forms beta pleated sheets.