This cocoon shell-associated protein inhibits trypsin Activity by forming a low-dissociation complex with trypsin. May play an important part in regulating proteolytic activity in the silk gland or protecting silk proteins from degradation during histolysis. Monomer. Expressed exclusively in the middle silk gland. Expression differentially regulated in the middle silk glands during the final stage of larval growth with highest expression before the onset of spinning. Has an isoelectric point of 4.3.