Nucleophosmin (NPM), also named B23 or NO38, is a major nucleolar protein which is 20 times more abundant in tumor or proliferating cells than in normal resting cells. NPM has been implicated in several distinct cellular functions, including assembly and transport of ribosomes, cytoplasmic/nuclear trafficking, regulation of DNA polymerase alpha activity, centrosome duplication and molecular chaperoning activities. The NPM is also known for its fusion with the anaplastic lymphoma kinase (ALK) receptor tyrosine kinase. The NPM portion contributes to transformation by providing a dimerization domain, which results in activation of the fused kinase.