The transforming growth factor-beta (TGF-beta) superfamily plays a cen
tral role in the specification and patterning of cells in the early em
bryo. Several years ago, the TGF-beta s were shown to signal through s
erine/threonine receptor kinases. Now, with the identification of Smad
proteins, we can trace the TGF-beta signal transduction pathway from
the receptors into the nucleus.