A cDNA, named DD4, was identified in the prawn Penaeus japonicus in a search for genes that were expressed during calcification of the crustacean exoskeleton. DD4 transcripts were detected in the epidermal cells underlying the exoskeleton specifically during the postmolt stage, when the calcification takes place. in the DD4 cDNA an open reading frame of 542 amino acids was found. The deduced protein was acidic and proline-rich, and exhibited similarity to the Drosophila Ca2+-binding protein calphotin in the amino acid sequence and composition. The DD4 cDNA was expressed in Escherichia coli to characterize Ca2+- binding of the encoded protein, and Ca2+ was found to bind to a central segment of 186 amino acids. The DD4 protein is suggested to play a role in the calcification of the crustacean exoskeleton.