Insect development and metamorphosis are regulated by two major hormones,juvenile hormone and ecdysteroids.Despite being the key regulator of insect developmental transitions,the metabolic pathway of the primary steroid hormone,20-hydroxyecdysone(20E),especially its inactivation pathway,is still not completely elucidated.A cytochrome P450 enzyme,CYP18A1,plays a key role in insect steroid hormone inactivation through 26-hydroxylation.Here,we identified two CYP18(BmCYP18A1 and BmCYP18B1)orthologs in the lepidopteran model insect,Bombyx mori.The BmCYP18A1 gene was predominantly expressed in the middle silk gland(MSG)while BmCYP18B1 expressed ubiquitously in B.mori.BmCYP18A1 was induced by 20E in vitro,suggesting its role in 20E metabolism.Using the binary Gal4/UAS transgenic system,we ectopically overexpressed BmCYP18A1 in a MSG-specific manner with a Sericin1-Gal4(Ser-Gal4)driver or in a ubiquitous manner with an Actin3-Gal4(A3-Gal4)driver.Ectopic overexpression of BmCYP18A1 in MSG or in all tissues resulted in developmental arrestment of transgenic animals during the final instar larval stage.The 20E titers in the transgenic animals expressing BmCYP18A1 were lower compared to the levels in the control animals.Although the biological significance of MSG-specific expression of BmCYP18A1 is unclear,our results provide the first evidence that BmCYP18A1,which is conserved in most arthropods,is involved in a tissue-specific steroid hormone inactivation in B.mori.