Author response image 1. Nodal signaling is elevated in lefty2 mutants.

Smad2 is the intracellular effector of Nodal signaling and is phosphorylated when activated (Schier 2009). We measured phosphorylated Smad2 (pSmad2) by immunostaining as described in a forthcoming manuscript (Rogers et al. submitted). Representative cross-sections from pools of 3-6 embryos at 50% epiboly stage are shown. Left column: nuclei are labeled with Sytox green DNA stain. Middle column: α -pSmad2 immunostaining. Right column: magnified view of embryo margins highlighted in middle column with white boxes.