Involved in pre-mRNA splicing as component of the spliceosome (PubMed:11991638, PubMed:28502770, PubMed:28076346, PubMed:33220177). PPIases accelerate the folding of proteins. Catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides (PubMed:16595688). Catalyzes prolyl peptide bond isomerization in CDC40/PRP17 (PubMed:33220177). Plays an important role in embryonic brain development; this function is independent of its isomerase activity (PubMed:33220177). [protein]-peptidylproline (omega=180) = [protein]-peptidylproline (omega=0) Inhibited by Cyclosporin A. Identified in the spliceosome C complex (PubMed:11991638, PubMed:28502770, PubMed:28076346). Interacts with SNW1/SKIP (PubMed:16595688, PubMed:20368803, PubMed:33220177). Interacts with CDC40/PRP17; this interaction leads to CDC40 isomerization (PubMed:33220177). Interacts with RBM22 (PubMed:33220177). Ubiquitous, with the most abundant expression in heart and skeletal muscle. The disease is caused by variants affecting the gene represented in this entry. Belongs to the cyclophilin-type PPIase family. PPIL1 subfamily.