Plays a critical role in both constitutive and enhancer-dependent splicing by mediating protein-protein interactions and protein-RNA interactions required for accurate 3'-splice site selection. Recruits U2 snRNP to the branch point. Directly mediates interactions between U2AF2 and proteins bound to the enhancers and thus may function as a bridge between U2AF2 and the enhancer complex to recruit it to the adjacent intron. Interacts (via RS domain) with PHF5A (via N-terminus) (By similarity). Identified in the spliceosome C complex. Heterodimer with U2AF2. Interacts with ZRANB2. The C-terminal SR-rich domain is required for interactions with SR proteins and the splicing regulators TRA and TRA2, and the N-terminal domain is required for formation of the U2AF1/U2AF2 heterodimer. The gene represented in this entry may be involved in disease pathogenesis. Mutation altering U2AF1 function in the context of specific RNA sequences can lead to aberrant alternative splicing of target genes, some of which may be relevant for MDS pathogenesis. Interacts with U2AF2 and stimulates U2AF splicing activity in vitro. Less efficient than isoform 1. Produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Belongs to the splicing factor SR family.