Tumor metastasis is the leading cause of death in cancer patients. From a series of tumor cohort studies, low expression of Nm23/NDP kinase has been correlated with poor patient prognosis and survival, lymph node infiltration, and histopathological indicators of high metastatic potential in a number of cancer types, including mammary and ovarian carcinomas and melanoma. In other tumor types, no correlation has been established. Transfection of Nm23/NDP kinase cDNA into highly metastatic breast, melanoma, prostrate and squamous cell carcinomas, and colon adenocarcinoma cells significantly reduced the metastatic competency of the cells in vivo. In culture, cell motility, invasion, and colonization were inhibited, whereas tumorigenicity and cellular proliferation were not affected, indicating that Nm23/NDP kinase acts as a metastasis suppressor.