 The kinetic model assumes that Vg0 would remain as monomers ready to heterodimerize with Nodal, which is a major assumption of the model and should be mentioned. Vg0 alone would be expected to homodimerize if Nodal is not present. Do the authors have evidence for Vg0 remaining as a monomer in the absence of Nodal? Non-reducing Western blot conditions could provide evidence for this.  