The present invention relates to the novel isoforms, RXR.alpha.2 and RXR.alpha.3, of nuclear receptor RXR.alpha.. Unlike known isoform RXR.alpha.1, the transcriptional activation functions of RXR.alpha.2 and RXR.alpha.3 are augmented by SRC-1. The present invention provides methods for evaluating the function of regulating augmentation by co-activators of these RXR.alpha. isoforms, and screening methods based on these evaluation methods. By controlling the interaction between isoforms and co-activators, transcription-controlling activity can be regulated in an isoform-specific manner.