Silk fibroin elementary unit consists in a disulfide-linked heavy and light chain and a p25 glycoprotein in molar ratios of 6:6:1. This results in a complex of approximately 2.3 MDa. Produced exclusively in the posterior (PSG) section of silk glands, which are essentially modified salivary glands. It is unclear whether the N-terminal residue of the mature protein is Ala-17 or Gly-18.