Many cancer cells contain numerous copies of small DNA circles that are not part of the chromosomes. This unusual ‘extrachromosomal DNA’ (ecDNA) carries cancer-promoting genes and has been associated with tumour drug resistance and poor patient outcome. The presence and amount of ecDNA, as well as the specific gene sequences each circle contains, vary substantially from one cancer cell to the next even within the same patient and are important factors that can influence tumour growth. However, the lack of a method for directly observing the behaviour of ecDNA in living cells has limited our ability to understand how it contributes to cancer progression.
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