NPM1 and acute myeloid leukemia: AML is a very heterogeneous disease and has been divided into several subgroups based on recurrent cytogenetic alterations [e.g., t(15;17)(q24.1;q21.2), inv(16)(p13.1q22), or t(8;21)(q22;q22.1)] and mutations (e.g., in NPM1, TP53, or CEBPA; refs. 1–3).