In a study of 226 patients with CMML, 6% harbored the SF3B1 mutation (mostly the K700E variant), with this mutation being strongly associated with the presence of ≥15% RS and mutually exclusive with mutations in other splicing genes, such as SRSF2 and U2AF1. Interestingly, these other splicing mutations were also associated with the presence of RS, suggesting that this feature in CMML is not restricted to the presence of a mutant SF3B1 clone (9). This evidence concerns the gene U2AF1 and chronic myelomonocytic leukemia.