As our knowledge of the genetic underpinnings of MDS and AML have evolved, morphologic features, such as the presence/absence of ring sideroblasts or the number of dysplastic hematopoietic lineages, have become less critical for subclassification to the point where the identification of a single gene rearrangement (e.g., MECOM, MLL/KMT2A) could warrant a diagnosis of AML rather than one of MDS. This evidence concerns the gene MECOM and acute myeloid leukemia.