Cyclophilins are ubiquitously expressed proteins that bind to prolines and can 
catalyse cis/trans isomerization of proline residues. There are 17 annotated 
members of the cyclophilin family in humans, ubiquitously expressed and 
localized variously to the cytoplasm, nucleus or mitochondria. Surprisingly, all 
eight of the nuclear localized cyclophilins are found associated with 
spliceosomal complexes. However, their particular functions within this context 
are unknown. We have therefore adapted three established assays for in vitro 
pre-mRNA splicing to probe the functional roles of nuclear cyclophilins in the 
context of the human spliceosome. We find that four of the eight 
spliceosom-associated cyclophilins exert strong effects on splicing in vitro. 
These effects are dose-dependent and, remarkably, uniquely characteristic of 
each cyclophilin. Using both qualitative and quantitative means, we show that at 
least half of the nuclear cyclophilins can act as regulatory factors of 
spliceosome function in vitro. The present work provides the first quantifiable 
evidence that nuclear cyclophilins are splicing factors and provides a novel 
approach for future work into small molecule-based modulation of pre-mRNA 
splicing.