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. 2020 Dec 11;10(12):346. doi: 10.3390/life10120346

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© 2020 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Panel (a) shows standard base pairs in contemporary biology. Panel (b) shows an Expanded Genetic Information System containing 8 non-standard bases arranged as 4 non-standard pairs; K–X, isoC–isoG, S–B and Z–P.