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. 2021 Sep 9;8:693641. doi: 10.3389/fvets.2021.693641

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Copyright © 2021 Cao, Wang, Ge, Zhang, Huang and Sun.

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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A comprehensive classification of extrachromosomal circular DNAs (eccDNAs). We divide the general eccDNAs into two categories: narrow sense eccDNAs with length < 100 kb and ecDNAs common in tumors with size ranging from several 100 kb to several megabases. Small polydisperse circular DNA (spcDNA) was an obsolete concept to commonly characterize small eccDNAs with repetitive sequences. The length of extrachromosomal rDNA circles (ERCs) and t-circles is larger than that of spcDNA, which means that some ERCs and t-circles were not covered by spcDNA. The deletion of large genomic fragments could be circularized into episomes and subsequently polymerized into double-minutes.