Fig. 7. Model for potential mechanisms by which TBX5 functions to control embryonic cardiac cell cycle progression. Schematic of embryonic cardiac cell cycle shown in left hand panel and potential roles for TBX5 in regulating G₁/S progression. (1) TBX5 may function to induce the expression of a growth factor, for example EGF or FGF required for cell cycle G₁ progression. (2) TBX5 may function to regulate E2F activity by controlling the phosphorylation state of pRB. (3) TBX5 could also negatively regulate general cell cycle inhibitors such as p27^Xic. (4) Alternatively TBX5 may function to regulate the expression of a key component of the CDK-cyclin complexes required for G₁/S progression.