Silk is a natural protein fiber, some forms of which can be woven or knitted into textiles. The protein fiber of silk is composed mainly of fibroin (75-80%) and a smaller coating of sericin (20-25%). It is most commonly produced from the cocoons of certain insect larvae. The best-known silk is obtained from the cocoons of the larvae of the mulberry silkworm Bombyx mori, which are reared in captivity (sericulture).