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. 2012 Jun 4;7(6):e38393. doi: 10.1371/journal.pone.0038393

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This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration, which stipulates that, once placed in the public domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose.

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Inorganic phosphate (P_i) is a component of mineral hydroxyapatite (HAP), while pyrophosphate (PP_i) is a potent inhibitor of HAP crystal precipitation and growth. The enzyme tissue nonspecific alkaline phosphatase (TNAP) hydrolyzes PP_i to release ionic P_i, creating conditions conducive for mineralization. Local PP_i is increased by the functions of the progressive ankylosis protein (ANK) and ectonucleotide pyrophosphatase phosphodiesterase 1 (NPP1), which act to keep the mineralization process in check.