Component of the PRP19-CDC5L complex that forms an integral part of the spliceosome and is required for activating pre-mRNA splicing. Participates in AID/AICDA-mediated somatic hypermutation (SHM) and class-switch recombination (CSR), 2 processes resulting in the production of high-affinity, mutated isotype-switched antibodies (PubMed:32484799).