On the whole, the transcription factors and signalling genes are deeply conserved, but the proteins in the secretome are highly derived and rapidly evolving.[1] engrailed serves to demark the edge of the shell field; dpp controls the shape of the shell, and Hox1 and Hox4 have been implicated in the onset of mineralization.[16] In gastropod embryos, Hox1 is expressed where the shell is being accreted;[17] however no association has been observed between Hox genes and cephalopod shell formation.[18] Perlucin increases the rate at which calcium carbonate precipitates to form a shell when in saturated seawater;[19] this protein is from the same group of proteins (C-type lectins) as those responsible for the formation of eggshell and pancreatic stone crystals, but the role of C-type lectins in mineralization is unclear.[19] Perlucin operates in association with Perlustrin,[19] a smaller relative of lustrin A, a protein responsible for the elasticity of organic layers that makes nacre so resistant to cracking.[20] Lustrin A bears remarkable structural similarity to the proteins involved in mineralization in diatoms – even though diatoms use silica, not calcite, to form their tests![21]