The field of the invention is inducible gene expression systems wherein expression is controlled by a novel ecdysone or ultraspiracle receptor or its derivatives. The field particularly relates to the isolation and characterization of nucleic acid and polypeptides for a novel ecdysone receptor and a novel ultraspiracle receptor. The nucleic acid and polypeptides are useful in novel gene expression systems inducible with ecdysone or derivative receptor agonists.