Frzb expression and function. (A) Double in situ hybridization localizing frzb (dark blue) and chordin (brown) messages. frzb mRNA can be seen to be transcribed in the head endomesoderm of the organizer, but not in the notochord (where chordin is expressed). (B) Microinjection of frzb mRNA into the marginal zone leads to the inhibition of trunk formation. (From Leyns et al. 1997; photographs courtesy of E. M. De Robertis.)