Two human NM23 genes have been identified: NM23.H1 and NM23.H2 coding for the A and B subunit of a nucleoside diphosphate kinase (NDPK), respectively. NM23.H1 gene has been proposed as a suppressor of metastatic ability in tumor cells, NM23.H2 is identical to the c-myc transcription factor, PuF. The NM23 coding sequence is strongly preserved through different species. Indirect evidence of various types has been accumulated and seems to support an implication of NM23 in cell proliferation. This report shows that the NM23 gene expression is strictly related to the growth state of the cells. Two different in vitro systems (human peripheral blood lymphocytes and human breast epithelial cell line MCF-10A) and one in vivo (human primary infiltrating ductal breast carcinomas) system have been investigated. The mRNA is present in PHA-stimulated peripheral blood lymphocytes, whereas it is nearly undetectable in their resting counterparts. The level of the NM23 gene expression parallels the fraction of cells incorporating thymidine (S-phase) in neoplastic mammary tissues. In synchronously cycling MCF-10A cells NM23.H1 mRNA reaches a maximum abundance in the S-phase and is absent or only present at very low levels during G0/G1 phase, whereas NM23.H2 is present in growth-arrested cells but is upregulated following serum growth stimulation.