Nucleophosmin (NPM, also known as B23, numatrin1 or NO38) is able to bind to many partners in distinct cellular compartments, including nucleolar factors, transcription factors, histones, proteins involved in cell proliferation (for example, DNA polymerase-alpha), mitosis, (for example, NUMA and NEK2A) and the response to oncogenic stress (for example, ARF and p53). NPM also associates with both DNA and RNA, and it has been reported to have endoribonuclease activity to ribosomal RNA (rRNA). Furthermore, it forms complexes with the second messenger PIP3 in the nucleus in response to anti-apoptotic factors. NPM takes part in various cellular processes, such as the regulation of cell growth, proliferation and transformation, the transport of pre-ribosomal particles and ribosome biogenesis, the response to stress stimuli, the maintenance of genomic stability through the control of cellular ploidy and the participation in DNA-repair processes, and the regulation of DNA transcription through modulation of chromatin condensation and decondensation events. NPM is also involved in regulating the activity and stability of crucial tumor suppressors such as p53 and ARF. Its expression rapidly increases in response to mitogenic stimuli, and increased amounts of the protein are detected in highly proliferating and malignant cells. NPM [1].