L67 is a potent and specific inhibitor of DNA ligase IIIα (LigIIIα) that preferentially targets mitochondrial LigIIIα resulting in mitochondrial dysfunction. L67 preferentially targets cancer cell mitochondria resulting in enhanced ROS production and caspase 1-dependent apoptosis. L67 in combination with PARP inhibitors decreases survival rate of therapy resistant breast cancer and leukemia cells.