BACKGROUND Nucleotide diphosphate kinase (NDP kinase) is a phosphate transfer enzyme involved in cell regulation and in animal development. Drosophila NDP kinase is the product of the abnormal wing disc (awd) developmental gene, a point mutation in which can produce the killer of prune (K-pn) conditional lethal phenotype. The highly homologous mammalian genes control metastasis and a human NDP kinase acts as a transcription factor. RESULTS The X-ray structure of the Awd protein prepared from Drosophila was solved at 2.4 A resolution by molecular replacement from the homologous Dictyostelium protein. Both are hexamers, and both have the same fold and the same active site. Subunit contacts differ as a result of sequence changes in the carboxy-terminal segment and in the loop that is the site of the K-pn mutation. CONCLUSIONS Regulatory properties of animal NDP kinases depend on interactions with other macromolecules, such as DNA and the product of the Drosophila prune gene. The Awd structure suggests an allosteric mechanism of action of NDP kinase where DNA is the effector and the protein undergoes a major conformational change, possibly dissociating to dimers.