r the γ-phosphate of a nucleoside or deoxynucleoside triphosphate, usually ATP, to a nucleoside or deoxynucleoside diphosphate, yielding the substrates of RNA and DNA synthesis. The genes, present in almost all organisms, code for polypeptide chains of about 150 residues with very similar sequences and folds. Nevertheless, the quaternary structure (reviewed in Lascu et al.1) is not conserved: most NDPkinases are hexamers, but tetramers are found in some bacteria. The tetramer is illustrated bythe X-ray structure of the Myxococcus xanthus enzyme, 2 the hexamer, by those of Dictyostelium, Drosophila, and several human and bovine isoforms (reviewed in Janin et al.3). The eukaryotic gene products are 10-12 residues longer at the C-terminus than in Myxococcus. Because major intersubunit contacts implicate these residues in the hexamer, short-chain bacterial NDP kinases were assumed to be tetramers like Myxococcus.We present here the 2.6Å X-ray structure of the enzyme from Mycobacterium tuberculosis, which has an even shorter polypeptide chain, and show that it forms a very stable hexamer despite the missing interactions.