Highest level of adult expression is in the thorax. Low expression levels are seen in larvae and pupae. Expressed at higher levels in adult males than females. Plasmodium-refractory mosquitoes have higher levels of expression than susceptible mosquitoes. Expression levels are not increased after blood feeding or cold shock, septic wounding, bacterial injection, laminarin injection or CM-Sephadex bead injection. Belongs to the peptidase S1 family.