The organic constituent is mainly made up of polysaccharides and glycoproteins;[6] its composition may vary widely: some molluscs employ a wide range of chitin-control genes to create their matrix, whereas others express just one, suggesting that the role of chitin in the shell framework is highly variable;[2] it may even be absent in monoplacophora.[3] This organic framework controls the formation of calcium carbonate crystals[9][10] (never phosphate,[11] with the questionable exception of Cobcrephora[12]), and dictates when and where crystals start and stop growing, and how fast they expand; it even controls the polymorph of the crystal deposited,[7][13] controlling positioning and elongation of crystals and preventing their growth where appropriate.[6]