U2AF1 and essential thrombocythemia: Given the very small numbers, it is difficult to draw definitive conclusions regarding the co-occurrence of other additional mutations, however, signaling mutations seem more frequent in SRSF2- and U2AF1-mutated MF, but not ET and the absence of other additional mutation is less frequent in SRSF2-mutated ET (24% vs. 50–60%), but not MF.