We have defined a molecular signature for identifying responders to azacytidine therapy, characterized by the presence of mutations in DNMT3A, TET2, EZH2, or U2AF1. In this context, mutations in TET2 have previously been associated with responses to HMAs in MDS, or in AML with 20–30% blasts [7,21]. The gene discussed is U2AF1; the disease is myelodysplastic syndrome.