Fig. 3. Blocking both Smad1 and Smad2 signals leads to efficient neural induction in Xenopus ventral ectoderm. (A) At neurula stages, the truncated activin receptor tActRIB did not induce and tBMPRIA only weakly induced the neural markers Sox2 and Sox3. When the two truncated receptors were co-expressed, the neural markers were induced strongly to a level similar to that induced by the truncated type II receptor tActRIIB. One nanogram of each RNA was used. (B) Co-expression of tActRIB (1 ng) with Smad6 (0.1 ng) or VP16-Msx1 (0.1 ng) in ventral animal cells led to induction of the neural markers Sox2 and Sox3 in ventral ectoderm of frog neurulae.