NPM1 and acute myeloid leukemia: While in several cohorts, clonal AML evolution at disease recurrence was not observed [31,33,34,35,36,37,38,39,42,43,48,49,52,53,55,57,58,59,60,63,64,65,66,67,68,69], in the remaining studies the frequencies of patients experiencing AML relapse with undetectable NPM1 mutations were extremely variable, ranging from 1% in the prospective trial by Ivey et al. [54] to 25% documented in smaller series [41,94].