A method of screening for selective inhibitors or activators Smad protein function is disclosed. In one embodiment, the invention comprises the steps of (a) obtaining a phosphorylated Smad protein or protein complex, (b) allowing the phosphorylated Smad protein or complex to interact with a target protein or peptide in the presence of a test compound, and (c) analyzing the binding of the phosphorylated Smad complex or protein and the target protein or peptide, wherein a perturbation of binding indicates that the test compound is an inhibitor or activator of Smad-target protein interaction.