In eukaryotic cells, replicated DNA strands remain physically connected until 
their segregation to opposite poles of the cell during anaphase. This "sister 
chromatid cohesion" is essential for the alignment of chromosomes on the mitotic 
spindle during metaphase. Cohesion depends on the multisubunit cohesin complex, 
which possibly forms the physical bridges connecting sisters. Proteolytic 
cleavage of cohesin's Sccl subunit at the metaphase to anaphase transition is 
essential for sister chromatid separation and depends on a conserved protein 
called separin. We show here that separin is a cysteine protease related to 
caspases that alone can cleave Sccl in vitro. Cleavage of Sccl in metaphase 
arrested cells is sufficient to trigger the separation of sister chromatids and 
their segregation to opposite cell poles.