Binds chitin and may serve as a framework constituent participating in shell formation. Inhibits aragonite precipitation and may regulate aragonite growth during shell layer formation. Does not affect calcite crystallization. Expression is confined to the prism and organic layers of the shell with no expression detected in the nacreous shell layer. Also expressed in the mantle edge, extrapallial fluid, hemolymph and, to a lesser extent, in the viscus (at protein level). In the mantle, localizes to inner epithelial cells of the outer fold and the outer epithelial cells of the middle fold at the bottom of the periostracal groove.