Nucleophosmin (NPM), also known as nucleolar phosphoprotein B23 or numatrin, is a protein that in humans is encoded by the NPM1 gene. NPM1 is associated with nucleolar ribonucleoprotein structures and binds single-stranded and double-stranded nucleic acids, but it binds preferentially G-quadruplex forming nucleic acids. It is involved in the biogenesis of ribosomes and may assist small basic proteins in their transport to the nucleolus. Its regulation through SUMOylation (by SENP3 and SENP5) is another facet of the protein's regulation and cellular functions. It is located in the nucleolus, but it can be translocated to the nucleoplasm in case of serum starvation or treatment with anticancer drugs. The protein was identified as a phosphoprotein. However, later other post-translational modifications for NPM1 were identified including acetylation and SUMOylation.