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. Levels are low until the penultimate larval instar, and peak in feeding larvae of this instar. Expression decreases again at the molt to the last larval instar. Levels rise rapidly after ecdysis when the larvae initiate feeding and reach a maximum in slowly mobile prepupae at the end of cocoon spinning. There is then a ten-fold drop which coincides with the start of silk gland degeneration. Found in 2 differentially glycosylated forms (29 kDa and 30 kDa).