TXN and neoplasm: This study a) confirmed that the biological function of Nm23-H1 is regulated by oxido-reduction; b) demonstrated that Cys109 of cellular Nm23, not in its active site, can be oxidized to cysteic acid, to produce disulfide bond with glutathione and inter- and intra-disulfide bonds with C4, C145 and C109; and that oxidized Nm23-H1 lost its enzymatic activity as well as ability to suppress tumor metastasis; and that oxidized Nm23-H1 interacts with thioredoxin reductase (TrxR) and is reduced by NADPH-TrxR-Trx system.