A pair of nuclear receptors, ecdysone receptor (EcR) and ultraspiracle (USP), heterodimerize and transduce ecdysteroid signals. The EcR and its nonsteroidal ligands are being developed for regulation of transgene expres-sion in humans, animals and plants. In mammalian cells, EcR:USP heterodimers can function in the absence of ligand, but EcR⁄retinoid X receptor (EcR:RXR) heterodimers require the presence of ligand for activa-tion.