Abstract: The present invention provides a binding domain of the protein Siah, which is known to be an important protein in the ubiquination pathway. The domain is capable of binding substances, co-factors and interactors of the Siah protein. The invention also provides a zinc-binding and a dimerisation domain of Siah. Also provided are methods of screening compounds that are capable of interacting with the binding domain, as well as methods of treating diseases including cancer, infertility, and inflammation.