Fig. 4. Msx1 expression is specified by a threshold concentration of Bmp. One-cell stage embryos were injected with a combination of 50 pg of Wnt5a mRNA and different amounts of CM-Bmp4 mRNA, which are indicated in the figure. Animal caps were dissected at stage 9 and the expression of msx-1, slug and histone H4 was analyzed by RT-PCR when sibling embryos reached the neurula stage 16. (A) Embryos and animal cap samples are shown. (B) Quantification of the gel shown in A.