Reef-building corals are widely distributed in shallow tropical and subtropical seawaters around the world and are one of main calcifying animals. Coral skeleton was made of aragonite of calcium carbonate. The skeleton of galaxy coral (Galaxea fascicularis) collected in Okinawa Prefecture was decalcified in the same way as other biominerals, and when the materials in the decalcified solution were subjected to SDS-PAGE, a 53 kDa protein named galaxin was found as a major band. Galaxin was the first matrix protein identified in corals. Some amino acid sequences were obtained. A cDNA encoding galaxin was cloned based on the partial amino acid sequences. The deduced amino acid sequence of galaxin (298 amino acid residues) has characteristic 10 times repeats of about 30 conserved amino acid residues. A recombinant galaxin was prepared by E. coli expression system as a fusion protein with maltose-binding protein (MBP). The fusion protein induced verterite in calcium carbonate calcification in vitro, but MBP alone gave only calcite. These results suggest that galaxin has an ability to interact with calcium carbonate.