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The NME (nm23/nucleoside diphosphate kinase) gene family in human is involved in the phosphorylation of nucleoside diphosphates and a variety of regulatory phenomena associated with development, oncogenic transformation, and metastasis. Here we report the cDNA sequence for a sixth member of this family, NME6. The cDNA sequence predicts a 186-residue protein that includes the characteristic active site motif of a nucleoside diphosphate (NDP) kinase, as well as the other residues previously identified as crucial for nucleotide binding and catalysis. The NME6 protein sequence is only 34-41% identical to the five previously reported human NME proteins, and is similarly related to prokaryotic and primitive eukaryotic NDP kinases. Compared to typical proteins of this family such as NME1 and NME2, NME6 has three additional residues located in the Kpn loop, and a 22-residue extension at the COOH-terminal. Using radiation hybrid mapping, the NME6 gene was localized to chromosome 3p21.3. The 1.3-kb transcript of NME6 is expressed at a moderately low level in many human tissues, and is most abundant in kidney, prostate, ovary, intestine, and spleen. Homologous cDNAs were also cloned and sequenced for rat and mouse. The sequence of the first 171 residues of the mouse homologue (Nm23-M6) is 94% identical to the deduced human NME6 protein.