Fig. 1. Sister-chromatid cohesion is required for accurate chromosome segregation. During a mitotic cell cycle, the genome is duplicated in S phase and each identical copy is then segregated into the daughter cells. In eukaryotes, this process is complex owing to the fragmentation of the genome in several chromosomes. Eukaryotic cells have evolved a mechanism, termed sister-chromatid cohesion, that keeps the two copies of a chromosome (sister chromatids) together from the moment of duplication to the onset of anaphase. This mechanism ensures the accurate segregation of one and only one copy of each chromosome to each daughter cell. When sister-chromatid cohesion is defective, mitotic processes such as chromosome biorientation and chromosome segregation are disrupted, resulting in aneuploidy, a hallmark of most cancers.