Pax6 is misregulated in Foxg1-/- cells. (A,B) Pax6 immunofluorescence on coronal sections through the telencephalon of wild-type (wt) (A) and Foxg1-/- (B) embryos at E12.5. The characteristic lateralhigh to mediallow gradient of Pax6 expression in the telencephalon is observed in wild-type (A) but not in mutant embryos (C). (C,G) Pax6 immunofluorescence on coronal sections through the telencephalon of a Foxg1+/+↔Foxg1+/- (C) control chimera and a Foxg1+/+↔Foxg1-/-experimental chimera (G). (D-F,H-J,K-M) Selected regions are shown at higher magnification and co-labelled for β-gal expressed by Foxg1+/- (D-F) or Foxg1-/- cells (H-J,K-M). Throughout the whole telencephalon of control chimeras (D-F) and the dorso-medial telencephalon of experimental chimeras (K-M), β-gal+ cells (arrows) express Pax6 at levels indistinguishable from those in adjacent β-gal- cells. In the dorso-lateral telencephalon of experimental chimeras (H-J), however, Foxg1-/- (β-gal+) cells display markedly lower levels of Pax6 than their neighbouring wild-type (β-gal-) cells. ET, eminentia thalami; PT, prethalamus.