Xenopus Dickkopf (Dkk) was initially discovered as a Wnt antagonist that plays an important role in head formation. By far, four members of Dkk have been identified in mammals. Each Dkk molecule contains two conserved cysteine-rich domains which are separated by a 50-55 amino acid linker region. Dkk4 is a secreted glycoprotein and has three immune-reactive sites at 15-17kDa, 30-32kDa and at 40kDa.