Fig. 2. The Smad family. Diagrammatic representation of the three subfamilies of Smads. The protein diagrams are arbitrarily aligned relative to their C-termini. The MH1 domain is coloured in blue and the MH2 domain in green. Selected domains and sequence motifs are indicated as follows: -helix H2, L3 and H3/4 loops, ß-hairpin, the unique exon 3 of Smad2 (ex3), NLS and NES motifs or putative (?) such motifs, the proline-tyrosine (PY) motif of the linker that is recognised by the Hect domain of Smurfs, the unique SAD domain of Smad4 and the SSXS motif of R-Smads with asterisks indicating the phosphorylated serine residues.