Fig. 8. The distinct temporal roles of Gli3R in regulating mouse mes/r1 development. (A) Time period of Gli3 expression, prenatal tectum and cerebellum phenotype and ectopic Fgf8 expression. Note that in Gli3^-/- mutants, a domain (X) forms between the tectum and cerebellum that is not properly specified as cerebellum (Cb), isthmus (Is) or inferior colliculus (Ic). Sc, superior colliculus. See Discussion for details. (B) High levels of Shh (lower pathway) regulate mes/r1 growth through induction of proliferation via Gli2A and/or by inhibition of cell death through Gli3R. Low levels of Shh (upper pathway) do not induce proliferation, but modulate cell death and proliferation [induced by unknown signal (X)] through the regulation of Gli3R levels.