Because the tumor cell outer membrane is impermeable to high molecular weight proteins such as catalase, this estimate of H2O2 formation probably reflects only the proportion of the total H2O2 pool present extracellularly; thus, it is not possible to make a direct stoichiometric comparison between total cyanide-resistant O2 consumption and H2O2 formation, even under identical experimental conditions. Here, CAT is linked to neoplasm.