Apoptosis is a common mode of programmed cell death occurring during development as well as in many pathological conditions, in which the cell plays an active role in its own demise. Although the morphological and biochemical hallmarks of apoptosis are conserved across phyla and cell type, the mechanism(s) of apoptosis is unknown. However, data recently published demonstrate that expression of the anti-apoptotic genebcl-2decreases the net cellular generation of reactive oxygen species, and that reactive oxygen species serve as mediators of apoptosis in at least some cases.