Methods are provided for identifying agents that modulate signaling
mediated by transforming growth factor beta (TGF-.beta.) and members of
the TGF-.beta. family, such as bone morphogenic protein (BMP). Such
agents may be identified using screens that evaluate candidate agents for
the ability to modulate Smad protein degradation. Agents identified as
described herein may be used to augment or inhibit signaling mediated by
one or more TGF-.beta. family members in a variety of cell types and for
therapeutic purposes.