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. 2021 Feb 12;10:627891. doi: 10.3389/fonc.2020.627891

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Copyright © 2021 Ma, Su and Li

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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The number of publications by year and the overall trend consistently kept gradually increasing from 1998 to 2016, and the trend was to be relatively stable in the past 3 years.