Figure 4.

(A) Dose dependence of the effect of dominant-negative FGF receptor (XFD) on Krox-20 expression induced by Xlim-1/3m plus Xbra. At the 3m-to-Xbra ratio used, Otx2 is suppressed but Krox-20 is not activated, possibly because the tissue is posteriorized too extensively. Low levels of XFD in the recipient cap (cap 2) allow Krox-20 expression, but high levels inhibit. (B) Restoration of Otx2 expression by XFD in combined caps. Caps injected with a high level of XFD mRNA were combined with caps coinjected with Xlim-1/3m and Xbra mRNAs. Eight combined caps were collected at equivalent stage 27/28 and assayed by Northern blot analysis. d50 is an inactive form of the FGF receptor that was used as control.