But they can also be devious. Because they aren’t under the same selective pressure as normal chromosomes, the number of ecDNA molecules can be highly variable

But they can also be devious. Because they aren’t under the same selective pressure as normal chromosomes, the number of ecDNA molecules can be highly variable between daughter cells. This is one of the potential resistance mechanisms for a tumor. If you have a drug that targets a gene in the ecDNA, the tumor could only keep clones with a small number of ecDNAs during treatment. Then after the drug has cleared, the number of ecDNAs can then rise again. When you lose the default 2 copy state, natural selection can follow non-Mendelian patterns.