RNA-binding protein that acts as a pre-mRNA splicing factor (PubMed:15694343, PubMed:31271494, PubMed:28437394, PubMed:28302793, PubMed:24795046). Acts by promoting exon inclusion via regulation of exon cassette splicing (PubMed:31271494). Also acts as a transcriptional coactivator for steroid nuclear receptors ESR1/ER-alpha and ESR2/ER-beta, and JUN/AP-1, independently of the pre-mRNA splicing factor activity (By similarity). Interacts with NCOA6 and JUN. Interacts with ESR1 and ESR2, in the presence of estradiol (E2) (By similarity). Interacts with RSRC1 (via Arg/Ser-rich domain) (PubMed:15798186). Interacts with SF3B1 (PubMed:24795046). Interacts with ZNF106 (via N-terminus) (By similarity). Concentrated in nuclear speckles (PubMed:8227358). Colocalizes with the core spliceosomal snRNP proteins (PubMed:8227358). Widely expressed. Highly expressed in pancreas, skeletal muscle, lung and brain (PubMed:8227358). Expressed at intermediate level in kidney, liver and heart (PubMed:8227358). Aryl sulfonamide anticancer drugs, such as indisulam (E7070) or E7820, promote ubiquitination and subsequent degradation by the DCX(DCAF15) complex (PubMed:31693891, PubMed:28437394, PubMed:28302793). RBM39 degradation results in splicing defects and death in cancer cell lines (PubMed:31693891, PubMed:28437394, PubMed:28302793). Aryl sulfonamide anticancer drugs change the substrate specificity of DCAF15 by acting as a molecular glue that promotes binding between DCAF15 and weak affinity interactor RBM39 (PubMed:31686031, PubMed:31819272). Antibodies against RBM39 are present in sera from a patient with hepatocellular carcinoma who developed several autoantibodies. Belongs to the splicing factor SR family.