The nm23 gene was first identified by differential screening of melanoma cell lines of high and low metastatic potential. Two human homologues of the nm23 gene have been isolated and designated nm23-H1 and nm23-H2. The products of these genes have been identified as nucleoside diphosphate kinase A (NDPK-A) and NDPK-B, respectively. nm23-H1 and nm23-H2 are metastasis-suppressor genes implicated in the control of the metastatic process of alignant cells. Expression of nm23 protein has been investigated in a number of tumours and in several has been shown to indicate a less aggressive phenotype. Patients with breast cancer may have a good prognosis where the tumour is shown to express the human homologue of the nm23 gene product. This antibody recognizes Human nm23-H1 protein (NDPK-A) and has low crossreaction with nm23-H2 homologue (NDPK-B).