Inactivation of protective catalase seems to be sufficient to explain these initial effects as (i) subsequent intercellular ROS-dependent apoptosis signaling cannot occur in the presence of active membrane-associated catalase and as (ii) the apoptosis-mediating effect of photofrin-derived singlet oxygen is completely abrogated through the addition of exogenously added catalase and as (iii) catalase inactivation after photofrin treatment can be directly demonstrated through challenging photofrin-treated tumor cells with exogenous peroxynitrite. Here, CAT is linked to neoplasm.