Thirteen genes including <i>ASXL1, U2AF1, SRSF2, SF3B1</i>, and <i>ZRSR2</i> had significantly higher mutation frequencies in primary myelofibrosis (PMF) compared with essential thrombocythemia and polycythemia vera; this difference distinguished PMF from MPN and likened it to MDS. This evidence concerns the gene U2AF1 and acquired polycythemia vera.