Protein structural analysis of Dickkopf (Dkk) proteins. A,B. Amino acid sequence comparison of cysteine-rich domain 1 (A) and 2 (B) of vertebrate (human, Hs), urochordate (ascidian, Ci), echinoderm (sea urchin, Sp) and cnidarian (Hydra, Hm) Dkk124 and Dkk3 related proteins. C. Schematic representation of the domain structure of Dkk proteins throughout the animal kingdom. Accession numbers of all the Dkk sequences used are indicated in Tables S2 and S3. Hs, Homo sapiens; Ci, Ciona intestinalis; Hm, Hydra magnipapillata; Sp, Strongylocentrotus purpuratus.