U2AF1 and acute myeloid leukemia: Genetic analysis of the disease progression of MDS patients further revealed that U2AF1 mutations occur in the early clonal evolution5 and MDS patients carrying mutant U2AF1 (U2AF1mut) are at increased risk to progress to secondary acute myeloid leukemia (sAML).3,28,29 Defective hematopoiesis associated with U2AF1mut cannot be rescued by overexpression of wild-type U2AF1 (U2AF1wt) in mice30,31 suggesting that U2AF1mut might gain a new function or be neomorphic.