Mutations in mitochondrial DNA (mtDNA) are frequently observed in human tumors, but their impact on tumor cell behavior remains unclear. Using cytoplasmic hybrid (cybrid) technology, the authors replaced the endogenous mtDNA in a poorly metastatic mouse tumor cell line with that from a highly metastatic cell line and vice versa. They found that the recipient tumor cells acquired the metastatic potential of the transferred mtDNA. Specifically, the mtDNA conferring high metastatic potential contained G13997A and 13885insC mutations in the ND6 gene, which led to a deficiency in respiratory complex I activity and increased production of reactive oxygen species (ROS). Pretreatment of highly metastatic tumor cells with ROS scavengers suppressed their metastatic potential in mice. These findings indicate that mtDNA mutations can contribute to tumor progression by enhancing the metastatic potential of tumor cells. The study also suggests that ROS overproduction caused by mutated mtDNA may regulate metastasis through upregulation of antiapoptotic and angiogenic genes, such as MCL-1, HIF-1α, and VEGF. Additionally, the authors found that the metastatic potential of human tumor cells was enhanced by exchanging their endogenous mtDNA with mutant mtDNA that induces complex I-mediated ROS overproduction. This work provides evidence that mtDNA mutations can control the metastatic potential of certain tumor cells and highlights the potential therapeutic effectiveness of ROS scavengers in suppressing metastasis.Mutations in mitochondrial DNA (mtDNA) are frequently observed in human tumors, but their impact on tumor cell behavior remains unclear. Using cytoplasmic hybrid (cybrid) technology, the authors replaced the endogenous mtDNA in a poorly metastatic mouse tumor cell line with that from a highly metastatic cell line and vice versa. They found that the recipient tumor cells acquired the metastatic potential of the transferred mtDNA. Specifically, the mtDNA conferring high metastatic potential contained G13997A and 13885insC mutations in the ND6 gene, which led to a deficiency in respiratory complex I activity and increased production of reactive oxygen species (ROS). Pretreatment of highly metastatic tumor cells with ROS scavengers suppressed their metastatic potential in mice. These findings indicate that mtDNA mutations can contribute to tumor progression by enhancing the metastatic potential of tumor cells. The study also suggests that ROS overproduction caused by mutated mtDNA may regulate metastasis through upregulation of antiapoptotic and angiogenic genes, such as MCL-1, HIF-1α, and VEGF. Additionally, the authors found that the metastatic potential of human tumor cells was enhanced by exchanging their endogenous mtDNA with mutant mtDNA that induces complex I-mediated ROS overproduction. This work provides evidence that mtDNA mutations can control the metastatic potential of certain tumor cells and highlights the potential therapeutic effectiveness of ROS scavengers in suppressing metastasis.