The majority of cancer patients succumb to the consequences of metastatic disease. A correlation of increased nm23 expression to low metastatic potential has been established in several malignancies, based on published prognostic studies with tumour cohorts and transfection studies. Transfection of highly metastatic MDA-MB-435 human breast carcinoma cells with nm23-H1 cDNA resulted in a significant reduction in the metastatic potential in vivo. These transfections also showed inhibition of colonisation and motility, as well as morphological and biosynthetic differentiation in vitro. The biochemical mechanism of Nm23-H1 action, as well as the identity of proteins involved in its functional biochemical pathway, are still unknown. We summarise published and recent research concerning the role of the nm23 gene in metastasis and normal cellular differentiation.