Fig. 1. Domain structures of the ecdysone receptor components. The three isoforms of Drosophila EcR and the single Drosophila USP are shown, with the standard nuclear receptor regions indicated. Region C is the DNA-binding domain (DBD); region E, the ligand-binding domain (LBD). The three EcR isoforms are identical in sequence except in the A/B regions that are unrelated. The number of isoform-specific residues is shown for each EcR. The scale underneath (in residues) is aligned to EcR-B1. The residue mutated in EcR-F645A is indicated.