Hypothetical steps of genetic evolution in NPM1-mutated AML. In this scheme, the CD34⁺ hematopoietic stem cell (HSC) compartment (whether normal or leukemic) is shown in the central column, whereas its more differentiated CD34-negative progeny is shown in the right and left columns. The primary, driving NPM1 mutation (red dot) in an HSC causes transformation that leads to the “leukemic phenotype.” Other mutations (light blue dots), such as FLT3-ITD, occur later in clonal evolution. Leukemic cells in approximately 15% of NPM1-mutated AML can also acquire a chromosomal abnormality (X), whereas in 85% of cases they maintain a normal karyotype. Both later mutations and chromosomal abnormalities are usually expressed in a leukemic cell subclone whose size may vary from one patient to another. For simplicity, occurrence of the second mutation and a chromosomal abnormality in the same cells is not shown. According to the 2-hit hypothesis, only 2 mutations are indicated, but additional mutations may be involved. Light gray circles represent normal HSC and multipotent progenitors; and green circles indicate the CD34⁺ normal hemopoietic progenitor compartment where primary NPM1 mutation (red dot) and secondary mutations (blue dot) and/or chromosomal aberrations (X) occur, giving rise to the CD34⁻ leukemic bulk population.