Recent results from our group have shown that extracellular singlet oxygen generated through the interaction between cell-derived H2O2 and peroxynitrite [62] has the potential to inactivate membrane-associated catalase that protects tumor cells from intercellular ROS signaling [29,63,64] and thus reactivates ROS-dependent apoptosis induction selectively in tumor cells. Here, CAT is linked to neoplasm.