Expression of the antimetastatic gene nm23 in human breast cancer: an association with good prognosis. The nm23 gene was identified in murine melanoma cells, in which its expression is associated with the cells' metastatic potential. Expression of nm23 has been detected in human breast tumors by means of hybridization and immunocytochemistry. We measured nm23 mRNA in 71 patients with primary breast cancer and found variable levels of nm23 expression. The nm23 gene was expressed at higher levels in well-differentiated tumors (P less than .02). There was a significant inverse relationship between nm23 expression and nodal status (P less than .02). Expression of nm23 was positively associated with longer disease-free survival and overall survival, and the relationships were significant (P less than .002 and P less than .003, respectively). This study showed that nm23 expression in human breast cancer was associated with good prognosis and a lack of lymph node metastasis and suggests that the nm23 gene product may play an important role in suppressing the metastatic phenotype. 