Nucleophosmin 1 (NPM1), also known as nucleolar phosphoprotein B23 or numatrin, is a member of the nucleoplasmin family. Nucleophosmin (NPM) is a nucleolar phosphoprotein that plays multiple roles in ribosome assembly and transport, cytoplasmic-nuclear trafficking, centrosome duplication and regulation of p53. The NPM1 gene is frequently involved in chromosomal translocation, mutation and deletion. Mutations of the NPM1 gene leading to the expression of a cytoplasmic mutant protein, NPMc+, are the most frequent genetic abnormalities found in acute myeloid leukemias. Acute myeloid leukemias (AML) with mutated NPM1 have distinct characteristics, including a significant association with a normal karyotype, involvement of different hematopoietic lineages, a specific gene-expression profile and clinically, a better response to induction therapy and a favorable prognosis. In addition, NPM1 is a crucial gene to consider in the context of the genetics and biology of cancer. NPM1 is frequently overexpressed, mutated, rearranged and deleted in human cancer. Traditionally regarded as a tumour marker and a putative proto-oncogene, it has now also been attributed with tumour-suppressor functions.