Mutations in SRSF2 and U2AF1 are the most frequent splicing-associated mutations found in the more aggressive subtypes of MDS, including refractory anemia with excess blasts I (RAEB I) and RAEB II (Inoue et al. 2016). The MDS-associated spliceosome mutations identified thus far are heterozygous mutations as opposed to nonsense mutations, indicating that these mutations are neomorphic or dominant-negative (Inoue et al. 2016). This evidence concerns the gene U2AF1 and myelodysplastic syndrome.