As part of the spliceosome, plays a role in pre-mRNA splicing (PubMed:29360106). Probable inactive PPIase with no peptidyl-prolyl cis-trans isomerase activity (PubMed:20676357). Part of the activated spliceosome B/catalytic step 1 spliceosome, one of the forms of the spliceosome which has a well-formed active site but still cannot catalyze the branching reaction and is composed at least of 52 proteins, the U2, U5 and U6 snRNAs and the pre-mRNA. Recruited during early steps of activated spliceosome B maturation, it is probably one of the first proteins released from this complex as he matures to the spliceosome C complex. The disease is caused by variants affecting the gene represented in this entry. Belongs to the cyclophilin-type PPIase family. Despite the fact that it belongs to the cyclophilin-type PPIase family, a report has shown that it has probably no peptidyl-prolyl cis-trans isomerase activity.