Nucleophosmin is a nucleus protein which belongs to the nucleoplasmin family. It is a nucleolar phosphoprotein that shuttles between the nucleus and cytoplasm during the cell cycle. Nucleophosmin / NPM1 is involved in diverse cellular processes such as ribosome biogenesis, centrosome duplication, protein chaperoning, histone assembly, cell proliferation, and regulation of tumor suppressors TP53 / p53 and ARF. Subcellular localization of Nucleophosmin / NPM1 appears to be strongly correlated with NPM1 functions and cell proliferation. It is phosphorylated mainly at its central acidic domain by several upstream kinases, and its phosphorylation appears to be involved in regulating its functions in ribosome biogenesis and centrosome duplication. Nucleophosmin / NPM1 binds ribosome presumably to drive ribosome nuclear export. It is associated with nucleolar ribonucleoprotein structures and bind single-stranded nucleic acids. Nucleophosmin / NPM1 acts as a chaperonin for the core histones H3, H2B and H4. Nucleophosmin / NPM1 may act as a licensing factor to maintain proper centrosome duplication and that the Ran/CRM1 nucleocytoplasmic complex regulates local trafficking of Nucleophosmin / NPM1 to centrosomes by interacting through its nuclear export sequence motif.