As it has been previously noted, sister chromatids stay associated from S phase (when DNA is replicated to generate two identical copies, the two chromatids) until anaphase. At this point, the two sister chromatids separate and travel to opposite poles in the dividing cell. Genetic and biochemical studies in yeast and in egg's extracts in "Xenopus laevis" identified a polyprotein complex as an essential player in sister chromatids cohesion (see the review from Hirano in 2000). This complex is known as the cohesin complex and in "Saccharomyces cerevisiae" is composed of at least four subunits: Smc1p, Smc3p, Scc1p (or Mcd1p) and Scc3p. Both Smc1p and Smc3p belong to the family of proteins for the Structural Maintenance of Chromosomes (SMC), which constitute a group of chromosomic ATPases highly conserved, and form an heterodimer (Smc1p/Smc3p). Scc1p is the homolog in "S.cerevisiae" of Rad21, first identified as a protein involved in DNA repair in "S. pombe". These four proteins are essential in yeast, and a mutation in any of them will produce premature sister chromatid separation. In yeast, cohesin binds to preferential sites along chromosome arms, and is very abundant close to the centromeres, as it was shown in a study using chromatin immunoprecipitation.