Mutations of the nucleophosmin (NPM1) gene, encoding for a nucleolar multifunctional protein, occur in about one-third of adult acute myeloid leukemia (AML). NPM1-mutated AML exhibits unique molecular, pathological and clinical features, that led to its recognition as distinct entity in the 2017 World Health Organization (WHO) classification of myeloid neoplasms. Although WHO criteria for the diagnosis of NPM1-mutated AML are well established, its distinction from other AML entities may be sometimes difficult. Moreover, the percentage of blasts required to diagnose NPM1-mutated AML remains controversial. According to the European LeukemiaNet (ELN), determining the mutational status of NPM1 (together with FLT3) is mandatory for accurate relapse risk assessment. NPM1 mutations are ideal targets for measurable residual disease (MRD) monitoring since they are AML-specific, frequent, very stable at relapse and do not drive clonal hematopoiesis of undetermined significance. MRD monitoring by quantitative PCR of NPM1 mutant transcripts, possibly combined with the ELN genetic-based risk stratification, can guide therapeutic decisions at post-remission stage. Furthermore, immunohistochemistry can be very useful in selected situations, such as diagnosis of NPM1-mutated myeloid sarcoma. Herein, we present four illustrative cases of NPM1-mutated AML, with the aim to address important issues on the biology, diagnosis and therapy of this common form of leukemia.

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