The genetic material of cancer cells is unstable. For example, the number of chromosomes, which are the individual elements of packed DNA, is changed in so called aneuploidies. This imbalance in chromosomes, which often occurs early in tumor development, leads to cell stress and promotes disease. How this can happen is now shown by the discovery of a research team led by Zuzana Storchová at the Max Planck Institute of Biochemistry in Martinsried, reported in a groundbreaking study published in Nature Communications. An imbalance in an enzyme called MCM2-7 that is essential for DNA replication is likely to be responsible for this escalating genomic instability.