A number of drugs that work through nuclear receptors display an agonist response in some tissues and an antagonistic response in other tissues. This behavior may have substantial benefits since it may allow retaining the desired beneficial therapeutic effects of a drug while minimizing undesirable side effects. Drugs with this mixed agonist/antagonist profile of action are referred to as selective receptor modulators (SRMs). Examples include Selective Androgen Receptor Modulators ( SARMs ), Selective Estrogen Receptor Modulators ( SERMs ) and Selective Progesterone Receptor Modulators ( SPRMs ). The mechanism of action of SRMs may vary depending on the chemical structure of the ligand and the receptor involved, however it is thought that many SRMs work by promoting a conformation of the receptor that is closely balanced between agonism and antagonism. In tissues where the concentration of coactivator proteins is higher than corepressors , the equilibrium is shifted in the agonist direction. Conversely in tissues where corepressors dominate, the ligand behaves as an antagonist. 
Estrogen and progesterone are steroid hormones that play a pivotal role in the regulation of mammalian reproduction. One primary action of these hormones is to regulate the development and function of the uterus. These hormones act by regulating the transcription of specific genes in the uterus. The actions of these hormones are mediated by their specific hormone receptors. These receptors are nuclear transcription factors, whose transcriptional regulatory activity is mediated by the binding of the specific steroid to these molecules. Once these receptors bind hormone, they can bind to specific cis-acting sequences in the promoter region of responsive genes and regulate transcription of these genes. In the regulation of transcription, these receptors interact with specific cofactors to activate the transcriptional machinery. A second gene family, the Steroid Receptor Coactivator (SRC) family, has been identified that serves to modulate the transcriptional activity of the hormone receptors. To date, three members of the SRC family have been identified. During the last decade, gene targeting technology has been used to identify the role of these receptors in the regulation of reproduction and uterine biology.