Involved in pre-mRNA splicing as component of the spliceosome (PubMed:28502770, PubMed:28076346). Component of the PRP19-CDC5L complex that forms an integral part of the spliceosome and is required for activating pre-mRNA splicing. Identified in the spliceosome C complex (PubMed:28502770, PubMed:28076346). Component of the PRP19-CDC5L splicing complex composed of a core complex comprising a homotetramer of PRPF19, CDC5L, PLRG1 and BCAS2, and at least three less stably associated proteins CTNNBL1, CWC15 and HSPA8 (PubMed:20176811). Interacts directly with CTNNBL1 in the complex (PubMed:20176811). Belongs to the CWC15 family. Has been termed C11orf5, but is not the official C11orf5 as defined by HGNC.