RBM5 promotes exon 4 skipping of AID pre-mRNA by competing with the binding of U2AF65 to the polypyrimidine tract

► We identify RBM5 as a regulator factor promoting AID exon 4 skipping. ► A weak 3′-splice site of intron 3 is required for RBM5-stimulated exon 4 skipping. ► RBM5 directly interacts with intron 3 of AID pre-mRNA. ► RBM5 competes with U2AF65 for the binding to the polypyrimidine tract in vitro.