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. 2018 Dec 3;8:17537. doi: 10.1038/s41598-018-35557-3

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© The Author(s) 2018

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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How to compute the genus. (a) A chain with several bonds (in blue and orange) connecting various pairs of residues (black dots). (b) Chord diagram representing the same structure. (c) Parallel chords replaced by a single chord, and then – together with the backbone – replaced by ribbons, whose single boundary is shown in red. (d) After shrinking the backbone to a small circle, the ribbon diagram can be smoothly drawn on a surface of a torus, whose genus is g = 1.