Distribution of mutations in myeloid malignancies. Some somatic mutations occur with relatively equal frequency across the spectrum of myeloid malignancies, while other mutations are enriched in particular pathological subtypes. Higher frequencies of mutation are portrayed with darker shading, and lower frequencies in lighter shading, along the spectrum of MDS to MPN (top bar) and AML (bottom bar). AML ontogeny is represented as a spectrum of de novo AML (no antecedent hematologic disorder), secondary AML (antecedent MDS), and post-MPN AML (antecedent MPN). (A) TET2 is an important regulator of stem cell self-renewal and myelomonocytic differentiation, consistent with the ubiquitous presence of TET2 mutations throughout myeloid malignancies, and with its striking enrichment in diseases with marked monocytosis, such as CMML. (B) DNMT3A mutations can be identified in a broad range of myeloid diseases, but are most frequent in de novo AML. (C) The FLT3 internal tandem duplication (FLT3-ITD) mutation drives myeloblast proliferation without inducing significant morphologic dysplasia, consistent with its strong association with de novo AML, but not with MDS, MPNs, or secondary AML. (D) The JAK2-V617F mutation causes a myeloproliferative phenotype, characteristic of MPNs, such as essential thrombocythemia (ET), polycythemia vera (PV), and primary myelofibrosis (PMF). (E) Mutations in SF3B1 are strongly associated with the presence of ring sideroblasts. Combinations of mutations can drive the composite features of overlap diseases, such as refractory anemia with ring sideroblasts (RARS) associated with marked thrombocytosis (RARS-T), where ∼50% possess the JAK2-V617F mutation and 70% harbor SF3B1 mutations.¹¹  (F) EZH2 mutations are found most commonly across the MDS-MPN spectrum. Despite the adverse prognostic impact of EZH2 mutations in MDS, these mutations are exceptionally rare in AML, implicating mechanisms other than leukemic transformation for their negative clinical effect.