ALTHOUGH mitotic cyclins are well-known substrates for ubiquitin-mediated proteolysis at the metaphase-anaphase transition(1-4), their degradation is not essential for separation of sister chromatids(5-8); several lines of evidence suggest that proteolysis of other protein(s) is required, however(4,6,9-11). Here we report the anaphase-specific proteolysis of the Schizosaccharomyces pombe Cut2 protein, which is essential for sister-chromatid separation(12,13). Cut2 is located in the nucleus, where it is concentrated along the short metaphase spindle. The rapid degradation of Cut2 at anaphase requires its amino-terminal region and the activity of Cut9 (ref. 14), a component of the 20S cyclosome/anaphase-promoting complex (APC), which is necessary for cyclin destruction(3,4,11). Expression of non-degradable Cut2 blocks sister-chromatid separation but not cell-cycle progression. This defect can be overcome by grafting the N terminus of cyclin B onto the truncated Cut2, demonstrating that the regulated proteolysis of Cut2 is essential for sister-chromatid separation.