Fig. 6. Model for the regulation of cardiac morphogenesis by Tbx1 in the secondary heart field. We propose that Tbx1 exerts a dual role on the left side of the SHF. Tbx1 acts through a novel second pathway in the left cardiac precursor cells of the SHF via Pitx2. It regulates the maintenance of Pitx2c expression, originally activated in the left LPM by Nodal signaling, by interaction with Nkx2.5 to regulate cell proliferation or migration to ensure proper asymmetric cardiac morphogenesis (yellow box). This pathway complements the previously described non cell-autonomous Tbx1-Fgf8/10 pathway in the left SHF. The Tbx1-Fgf8/Fgf10 pathway is required for cell proliferation in the right and the left SHF.