Decreased Smad2 phosphorylation in Smad3 null mice. (A) The MH1 and MH2 domains in Smad2, conserved in other Smads, are connected by a less-conserved linker region. TGF-β and MAPKs signalling induces phosphorylation of certain sites in Smad2. (B) The level of activated Smad2 (carboxy-terminal phosphorylation) is not modified by Smad3 ablation. (C) However, in Smad3 null mice the linker region is less phosphorylated, suggesting a decrease in an intracellular regulatory signal of Smad2 (*P < 0.05, Student's t-test, n = 6 per genotype, each measured two to three times).