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Nucleoside diphosphate kinase homolog 5 (NDP kinase homolog 5, NDPk5, NM23-H5; Inhibitor of p53-induced apoptosis-beta, IPIA-beta): In human, mRNA for NDPk5 is almost exclusively found in testis, especially in the flagella of spermatids and spermatozoa, in association with axoneme microtubules, and may play a role in spermatogenesis by increasing the ability of late-stage spermatids to eliminate reactive oxygen species. It belongs to the nm23 Group II genes and appears to differ from the other human NDPks in that it lacks two important catalytic site residues, and thus does not appear to possess NDP kinase activity. NDPk5 confers protection from cell death by Bax and alters the cellular levels of several antioxidant enzymes, including glutathione peroxidase 5 (Gpx5).