Plays a role in mRNA splicing as a component of the U4/U6-U5 tri-snRNP, one of the building blocks of the spliceosome. May also bind to DNA. Identified in the spliceosome C complex. Component of the U4/U6-U5 tri-snRNP complex composed of the U4, U6 and U5 snRNAs and at least PRPF3, PRPF4, PRPF6, PRPF8, PRPF31, SNRNP200, TXNL4A, SNRNP40, DDX23, CD2BP2, PPIH, SNU13, EFTUD2, SART1 and USP39. Found in the nucleus of mitogen-activated peripheral blood mononuclear cells (PBMCs), tumor cells, or normal cell lines, but not in normal tissues except testis and fetal liver or in unstimulated PBMCs, suggesting preferential expression in proliferating cells. A number of isoforms may be produced. Ubiquitously expressed. Sumoylated with SUMO2. Causes an allergic reaction in human. Binds to IgE from atopic dermatitis (AD) patients. Identified as an IgE autoantigen in atopic dermatitis (AD) patients with severe skin manifestations. Belongs to the SNU66/SART1 family.