The invention relates to assays for screening for chemicals which affect the binding between proteins comprising an SH3 binding motif (such as Proline-rich Nuclear Receptor Co-regulatory protein (PNRC)) and nuclear receptor proteins. Compounds which affect the binding of PNRC to nuclear receptors can affect the expression of genes whose expression is under the control of nuclear receptors to which PNRC binds. PNRC interacts with the orphan receptors SF1 and ERR.alpha.1 in a ligand-independent manner and interacts with the ligand-binding domains of nuclear receptors such as ER, AR, PR, TR, BAR, and RXR in a ligand-dependent manner. A 23-amino acid region in the carboxy-terminal proline-rich region which contains an SH3-binding motif is sufficient for the interaction with nuclear receptors.