NPM1 (Nucleophosmin 1; also nucleolar protein 23) is a 40 kDa member of the nucleoplasmin family of proteins. NPM1 is found in the nucleus where it performs multiple functions. NPM1 normally associates with unduplicated centrosomes. After phosphorylation at Thr199, NPM1 dissociates from centrosomes, allowing for centrosome duplication. NPM1 also negatively regulates GCN5-dependent histone acetylation during mitosis. Human NPM1 is 294 amino acids (aa) in length. It contains a Met-rich region (aa 1 - 9), two acidic regions (aa 120 - 132 and 161 - 188) and two NLSs (aa 152 - 157 and 191 - 197). Phosphorylation occurs at Ser4/10/70/254. Splice variants show a deletion of aa 195 - 223, a two aa substitution for aa 258 - 294 and an 11 aa substitution for aa 288 - 294. Over aa 9 - 158, human NPM1 is 97% aa identical to mouse NPM1.