Involved in pre-mRNA splicing as component of the spliceosome (PubMed:33220177). PPIases accelerate the folding of proteins. It catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides (By similarity). Catalyzes prolyl peptide bond isomerization in CDC40/PRP17 (By similarity). 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. Interacts with SNW1/SKIP. Interacts with CDC40/PRP17; this interaction leads to CDC40 isomerization. Interacts with RBM22. Widely expressed in the developing cortex at 14.5 dpc. Mutant mice, in which PPIL1 is truncated at position 102, die before 12.5 dpc. Belongs to the cyclophilin-type PPIase family. PPIL1 subfamily.