The splicing factors hPrp 18 and hPrp 4 have a short region of homology that defines a high affinity binding site for USA CyP in each protein . ^^^ USA CyP forms separate , stable complexes with hPrp 18 and hPrp 4 in which the active site of the cyclophilin is exposed . ^^^ Our results lead to a model in which USA CyP is carried into the spliceosome in complexes with hPrp 4 and hPrp 18 , and USA CyP acts during splicing within these complexes . ^^^