NPM/B23 (also known as numatrin, nucleophosmin 1, nucleolar phosphoprotein B23) is a member of the nucleophosmin/nucleoplasmin family that shuttles between the nucleus and cytoplasm. This protein regulates the stability and transcriptional activity of p53 and acts as a molecular chaperone. NPM/B23 preferentially binds to denatured proteins and has been shown to stimulate DNA polymerase activity and control the duplication of centrosomes. The chaperone activity of NPM/B23 is regulated by protein kinase CK2 and promotes release of denatured substrates from NPM/B23. NPM/B23 is modified by phosphorylation onThr199. This protein has been shown to bind to RNA and DNA and interact with nucleolar proteins including nucleolin, protein P120, HIV-1 Rev protein, and hepatitis delta antigens. The Poly6191 antibody recognizes human phosphorylated NPM/B23 (Thr234/Thr237) and has been shown to be useful for Western blotting.