FIGURE 24-24 Effect of loss of TGF-β signaling. Binding of TGF-β, an anti-growth factor, to its receptor causes activation of Smad transcription factors. In the absence of effective TGF-β signaling owing to either a receptor mutation or a Smad mutation, cell proliferation and invasion of the surrounding extracellular matrix increase. See X. Hua et al., 1998, Genes & Dev. 12:3084.