U2AF1 and myelodysplastic syndrome: Huber et al. identified nine genetically defined, mutually exclusive hierarchical subgroups in MDS: (1) biallelic TP53 mutations (biTP53), (2) complex karyotype, (3) mutated RUNX1, (4) mutated ASXL1, (5) deletion of 5q [del(5q)], (6) mutated SF3B1, (7) mutations in U2AF1, SRSF2, and/or ZRSR2 (splicing factor-positive, SP+), (8) presence of at least one mutation in DNMT3A, TET2, or other genes recurrently mutated in MDS (splicing factor-negative with ≥1 mutation, SP−/≥1), and (9) complete absence of any of the above genetic markers (splicing factor-negative without mutations, SP−/0) [114].