In D. melanogaster a pulse of the steroid hormone ecdysone triggers the larval-to-adult metamorphosis, a complex process in which this hormone induces imaginal tissues to generate adult structures and larval tissues to degenerate. We show that the EcR gene encodes three ecdysone receptor isoforms (EcR-A, EcR-81, and EcR-B2) that have common DNA- and hormone-binding domains but different N-terminal regions. We have used isoform-specific monoclonal antibodies to show that at the onset of metamorphosis different ecdysone target tissues express different isoform combinations in a manner consistent with the proposition that the different metamorphic responses of these tissues require different combinations of the EcR isoforms. We have also determined temporal developmenta
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