Nucleoside diphosphate kinase NME4 associated with the inner mitochondrial membrane (Tokarska-Schlattner et al. 2008) catalyzes the reversible reaction of ribonucleoside and deoxyribonucleoside 5'-diphosphates with ATP to form the corresponding nucleoside 5'-triphosphates and ADP. The active form of the enzyme is a hexamer of NME4 polypeptides whose amino-terminal 33 residues, a mitochondrial translocation signal, have been removed (Milon et al. 2000). The substrate specificity of NME4 has not been examined in detail but is inferred to be broad like that of the homologous NME1, 2, and 3 kinases (Schaertl et al. 1998).