NPM1 and acute myeloid leukemia: Nucleophosmin is a nucleolar phosphoprotein that shuttles between the nucleus and the cytoplasm.3 Nucleophosmin protein-1 is thought to be involved in many cellular functions, including protein synthesis, DNA replication, and regulation of the cell cycle.3 The reported frequency of NPM1 mutations in patients with AML varies between 27% and 30% and is considered the most frequently identified molecular abnormality in CN-AML, occurring in 45% – 60% of cases.4,6,8,9 Four common mutations cause mutant NPM1 (one 4 bp duplication and three separate 4 bp insertions).