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. 2017 Feb 20;7:42720. doi: 10.1038/srep42720

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Copyright © 2017, The Author(s)

This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

PMC Copyright notice

The figure is reprinted by permission from Macmillan Publishers Ltd: Nature. Begent and Born, Nature, 227, 926–930 (1970), copyright 1970²⁴.