mRNA injection into the ventral blastomeres of Xenopus embryos of mRNA encoding 
Wnt pathway genes induces a secondary axis with complete head structures. To 
identify target genes of the pre-MBT dorsalization pathway that might be 
responsible for head formation in zebrafish, we have cloned zebrafish dickkopf1 
(dkk1), which is expressed in tissues implicated in head patterning. We found 
that dkk1 blocks the post-MBT Wnt signaling and dkk1 is a target of the pre-MBT 
Wnt signaling. Dkk1 overexpression in the prechordal plate suggests that Dkk1, 
secreted from the prechordal plate, expands the forebrain at the expense of the 
midbrain in the anterior neural plate. Furthermore, dkk1 acts in parallel to the 
homeobox gene bozozok and bozozok is required for the maintenance of dkk1 
expression. The nodal gene squint is also required for the maintenance of dkk1 
expression. Among the mutually dependent target genes of the pre-MBT Wnt 
signaling, dkk1 plays an important role in patterning the anterior head of 
zebrafish.