Fig. 8 A model for Msx function in cell cycle regulation. Msx1 indirectly upregulates cyclin D1 expression (red arrows), resulting in block of cell cycle exit (red bar) and subsequent differentiation. Although cyclin D1 can promote cellular proliferation (green arrow), Msx1 does not, presumably through its effects on other cell cycle components (broken red arrow). Thus, overexpression of Msx1 would block cell cycle exit without promoting proliferation, while loss of Msx1 activity might lead to premature cell cycle exit.