Fig. 8. Schematic drawing to show the inductive activity of Fgf8. In normal development, Fgf8 expression is induced at the interface of Otx2 and Gbx2 expression domains, overlapping with Gbx2. En1 and Pax2 are expressed both in the mesencephalon and the metencephalon, and they may be involved in the initiation of Fgf8 expression. In the metencephalic region, Fgf8 signal (red) is strong enough to support c-Irx2 and Gbx2 expression and to repress Otx2 expression, which may induce this region to develop as R1. Consequently, the alar plate of R1 differentiates as cerebellum. In contrast, in the mesencephalic region, Fgf8 signal may be too weak to repress Otx2 expression. The alar plate differentiates as tectum.