Sister chromatid cohesion mediated by the cohesin complex is essential for 
chromosome segregation during cell division. Using functional genomic screening, 
we identify a set of 26 pre-mRNA splicing factors that are required for sister 
chromatid cohesion in human cells. Loss of spliceosome subunits increases the 
dissociation rate of cohesin from chromatin and abrogates cohesion after DNA 
replication, ultimately causing mitotic catastrophe. Depletion of splicing 
factors causes defective processing of the pre-mRNA encoding sororin, a factor 
required for the stable association of cohesin with chromatin, and an associated 
reduction of sororin protein level. Expression of an intronless version of 
sororin and depletion of the cohesin release protein WAPL suppress the cohesion 
defect in cells lacking splicing factors. We propose that spliceosome components 
contribute to sister chromatid cohesion and mitotic chromosome segregation 
through splicing of sororin pre-mRNA. Our results highlight the loss of cohesion 
as an early cellular consequence of compromised splicing. This may have clinical 
implications because SF3B1, a splicing factor that we identify to be essential 
for cohesion, is recurrently mutated in chronic lymphocytic leukaemia.