Extra-chromosomal DNA consists of tiny, circular bodies of DNA that have escaped from the chromosomes to which they are normally bound. In roughly 20% of human cancer samples, some DNA breaks free in this way and scatters throughout the nucleus of a cell. Once free-floating, ecDNA is no longer subject to mitosis, the process by which chromosomes divide into two identical copies. This adds an element of unpredictability to how genes are inherited by daughter cells, allowing mutations to occur faster and on a more dramatic scale than Mendel's laws of inheritance would normally permit.
Paul Mischel of Stanford University began investigating ecDNA in cancer cells in 2012 and was the first to document the phenomenon. His work, published in Cell in April 2026, found that ecDNA fragments overwhelmingly contain information on defence mechanisms that cancer cells use to replicate rapidly and evade destruction—probably because cells carrying such ecDNA proliferate more easily. This may explain why many cancers evolve resistance to chemotherapy drugs at seemingly extraordinary rates.
Similar cellular tricks had previously been observed in bacteria and fungi, which use circular DNA to develop drug resistance.
Daughter cells can benefit from ecDNA only if these circular snippets weave themselves back into their chromosomes after mitosis. They do this with the help of "anchor proteins" that return the ecDNA to chromosomes and specific DNA sequences that allow reintegration. Howard Chang, chief scientific officer at AMGEN, collaborated with Mischel to identify these mechanisms. Mischel views the anchor proteins and reintegration sequences as prime targets for future treatment: disabling them should leave ecDNA adrift and strip tumour cells of their evolutionary advantage. Some suitable anchor proteins have already been identified. Clinical trials are pending.
Lillian Siu, president of the American Association for Cancer Research and an oncologist at the Princess Margaret Cancer Centre in Toronto, has cautioned that ecDNA is not the only factor behind aggressive cancers. Humdrum mutations caused by genome instability and defective DNA repair also contribute to ecDNA's appearance, which in turn may enhance such instability.
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