The human Nm23-H1/NDPK A and Nm23-H2/NDPK B encode for two subunits of nucleoside diphosphate kinase--a ubiquitous enzyme that transfers the terminal phosphates from ATP to (d)NDPs. Although having an 88% amino acid sequence identity and an already assigned biochemical role in the cell, the two subunits appear to have additional and distinctive cell functions. In particular, both subunits have been reported to be involved in tumor progression and metastasis. The aim of this study was to determine the specific, and potentially distinct, localizations of both subunits in tumor cells of different origin and differentiation and therefore to search for a possible link between their localization and the stage of disease. We used the GFP reporter system to analyze the ectopic expression of GFP-Nm23 proteins in head and neck tumor cell lines by fluorescent microscopy techniques. Our experiments revealed that GFP-fused Nm23-H1 and -H2 proteins display the same localization in transfected cells, regardless of their origin and differentiation status. The proteins are principally found in the cytosol and the endoplasmic reticulum. Moreover, some cells exhibit nuclear staining, which appears to be cell cycle-dependent.