Abstract: The present invention relates to a method of identifying an agent which alters (inhibits, enhances) activity of GDF-9. The method involves combining cells having a receptor for GDF-9 and a gene, wherein expression of the gene is regulated by binding of GDF-9 to the receptor; GDF-9; and an agent to be assessed. The combination produced is maintained under conditions appropriate for binding of GDF-9 to the receptors on the cells. The extent to which binding of GDF-9 to the receptors on the cells occurs is then determined, wherein binding of GDF-9 to the receptor to a lesser or greater extent in the presence of the agent to be assessed than in its absence, is indicative of an agent which alters GDF-9 activity.