A natural, protein polymer with a predominance of alanine and glycine amino acids. Silk is produced by silkworms that have fed on mulberry tree leaves. The body of a silkworm can retain proteins (i.e., raw material for silk) amounting to as much as 20% of its body weight. It is thought that silk may be altered, via genetic engineering of silkworms, to produce fibers of very high strength.