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. 2019 Jul 3;18:114. doi: 10.1186/s12943-019-1043-x

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© The Author(s). 2019

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided 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 Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

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Fig. 1 — CTCs and ctDNA in the peripheral blood. CTCs and ctDNA are the essential elements and are widely believed to be the cornerstones of the liquid biopsy. CTCs are cancer cells that circulate in the bloodstream after being naturally shed from the original or metastatic tumors, they are “seeds” of tumors and can lead to a new fatal metastasis; ctDNA is derived from apoptotic and necrotic tumor cells that release their fragmented DNA into the circulation and contain genetic defects identical to the original tumor cells