U2AF1 and myeloproliferative disorder: Recurrent somatic mutations in a subset of spliceosome genes (SF3B1, SRSF2, and U2AF1) are frequently identified (30–60% depending on disease phenotype) in patients afflicted with myelodysplastic syndromes (MDS), myeloproliferative neoplasms (MPN) such as myelofibrosis (MF), MDS/MPN overlap disorders such as chronic myelomonocytic leukemia (CMML), and secondary acute myeloid leukemia (sAML).1-8 These heterozygous and mutually exclusive mutations are enriched in hotspot codons in these 3’ splicing factor proteins resulting in aberrant alternative mRNA splicing in hematopoietic cells.