Cohesion between sister chromatids opposes the splitting force exerted by microtubules, and loss of this cohesion is responsible for the subsequent separation of sister chromatids during anaphase. We describe three chromosmal proteins that prevent premature separation of sister chromatids in yeast. Two, Smc1p and Smc3p, are members of the SMC family, which are putative ATPases with coiled-coil domains. A third protein, which we call Scc1p, binds to chromosomes during S phase, dissociates from them at the metaphase-to-anaphase transition, and is degraded by the anaphase promoting complex. Association of Scc1p with chromatin depends on Smc1p. Proteins homologous to Scc1p exist in a variety of eukaryotic organisms including humans. A common cohesion apparatus might be used by all eukaryotic cells during both mitosis and meiosis.