Focus issue: an expanding world for TGF-beta signaling.

The complexity of the signaling network that is initiated by members of the transforming growth factor-beta (TGF-beta) superfamily continues to increase. In addition, Smads, which were originally identified as key transducers of TGF-beta signals, are beginning to emerge as players in other TGF-beta-independent processes. Perspectives in this issue, as well as highlights from a recent meeting on TGF-beta and select resources from the Archives, spotlight this emerging complexity.