Fig. 4. Model of T-box transcription factor regulation of cardiac chamber development. Chamber myocardium is shown in red and non-chamber (primary) myocardium is shown in grey. Interactions between T-box genes in each of these compartments are indicated, with the relative expression levels of genes, as determined by positive and repressive interactions, reflected in the size of the lettering. Factor X (blue) is proposed to activate Tbx2 in non-chamber myocardium through repression of the Tbx2 inhibitory activity of Tbx20.