Involved in pre-mRNA splicing as component of the spliceosome (PubMed:28076346, PubMed:28502770). Component of the PRP19-CDC5L complex that forms an integral part of the spliceosome and is required for activating pre-mRNA splicing. As a component of the minor spliceosome, involved in the splicing of U12-type introns in pre-mRNAs (Probable). {ECO:0000269|PubMed:20176811, ECO:0000269|PubMed:28076346, ECO:0000269|PubMed:28502770, ECO:0000305|PubMed:33509932}. This is the function of CWC15 (CWC15 spliceosome associated protein, ENSG00000150316).