The steroid hormone 20-hydroxyecdysone (20E) initiates insect molting and metamorphosis through binding with a heterodimer of two nuclear recep-tors, the ecdysone receptor (EcR) and ultraspiracle (USP). Expression of the specific isoforms EcR-A and EcR-B1 governs steroid-induced responses in the developing cells of the silkworm Bombyx mori.