We hypothesize that a proportion of RS ≥15% (or less but with the presence of SF3B1 mutation) in the bone marrow of patients with CMML might define a subset of patients with biological characteristics that clearly differ from CMML without such features, and might have a clinical course closer to that of MDS-RS/SF3B1 and better than classical CMML. Here, SF3B1 is linked to chronic myelomonocytic leukemia.