SF1 and myelodysplastic syndrome: U2AF is a heterodimer composed of the small subunit U2AF1 and large subunit U2AF2 (U2AF65), among which U2AF1 is involved in the recognition of the AG at 3' ss and U2AF2 contacts the polypyrimidine tract.19, 20 U2AF1 binds to the AG dinucleotide interacting with both U2AF2 and SR protein to establish the earliest splicing complex E. Then, the U2AF complex replaces SF1 bind to the BPS with one of its subcomponents, SF3B1, to generate the splicing A complex.21 Mutations in spliceosome components have been detected in myelodysplastic syndromes (MDS).