U2AF1 and essential thrombocythemia: Mutations in RNA splicing factors were found in post-MPN AML with a frequency similar to PMF: SRSF2 mutations were observed in 17% of post-MPN AML (post-ET, -PV and -PMF AML, combined, p = 0.09 for PMF vs. AML), 8.1% for SF3B1 (p = 0.87 for PMF vs. AML), 9.5% for U2AF1 (p = 0.15 for PMF vs. AML), 1.1% for ZRSR2 and 5.3% for PRPF8 mutations (Table 1).