Sister chromatid cohesion is essential for chromosome segregation and is 
mediated by cohesin bound to DNA. Cohesin-DNA interactions can be reversed by 
the cohesion-associated protein Wapl, whereas a stably DNA-bound form of cohesin 
is thought to mediate cohesion. In vertebrates, Sororin is essential for 
cohesion and stable cohesin-DNA interactions, but how Sororin performs these 
functions is unknown. We show that DNA replication and cohesin acetylation 
promote binding of Sororin to cohesin, and that Sororin displaces Wapl from its 
binding partner Pds5. In the absence of Wapl, Sororin becomes dispensable for 
cohesion. We propose that Sororin maintains cohesion by inhibiting Wapl's 
ability to dissociate cohesin from DNA. Sororin has only been identified in 
vertebrates, but we show that many invertebrate species contain Sororin-related 
proteins, and that one of these, Dalmatian, is essential for cohesion in 
Drosophila. The mechanism we describe here may therefore be widely conserved 
among different species.