Sister chromatid cohesion depends on cohesin [1-3]. Cohesin associates with chromatin dynamically throughout interphase . During DNA replication, cohesin establishes cohesion , and this process coincides with the generation of a cohesin subpopulation that is more stably bound to chromatin . In mitosis, cohesin is removed from chromosomes, enabling sister chromatid separation . How cohesin associates with chromatin and establishes cohesion is poorly understood. By searching for proteins that are associated with chromatin-bound cohesin, we have identified sororin, a protein that was known to be required for cohesion . To obtain further insight into sororin's function, we have addressed when during the cell cycle sororin is required for cohesion. We show that sororin is dispensable for the association of cohesin with chromatin but that sororin is essential for proper cohesion during G2 phase. Like cohesin, sororin is also needed for efficient repair of DNA double-strand breaks in G2. Finally, sororin is required for the presence of normal amounts of the stably chromatin-bound cohesin population in G2. Our data indicate that sororin interacts with chromatin-bound cohesin and functions during the establishment or maintenance of cohesion in S or G2 phase, respectively.