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Issue 231: Extra-chromosomal DNA (ecDNA) is circular DNA generated due to chromosomal instability during cell replication. ecDNA has a high prevalence in tumour samples. Is it a by-product or a driving force of tumour progress where cells carrying ecDNA divide faster than those without ecDNA? Because of the lack of centromeres, the segregation of ecDNA copies between two daughter cells is random, which leads to a fast evolution of ecDNA heterogeneity among cells. We validated this fundamental hypothesis of random segregation in experiments, modeled the stochastic dynamics of ecDNA, and analysed its copy number distribution over time. This leads to multiple testable predictions to distinguish positively selected from neutral ecDNA, which are validated in various tumour cell lines as well as in patient data. To read more, read our paper here.