Zombie cells

Looks can be deceiving—even a cell that appears healthy may be doomed to die. Ekert et al. (page 835) report that cells lacking two important caspases can survive in the short term without a growth factor, but will succumb eventually. Thus, the normal job of these caspases may be to accelerate the dying process rather than control the commitment of cells to death.

The situation varies between organisms. In worms, deletion of CED-4 and CED-3 (the homologues of mammalian Apaf-1 and caspase-9) completely prevents programmed cell death. Some mice without Apaf-1 and caspase-9 die from neuronal overgrowth but others fare just fine. One possibility is that another caspase such as caspase-2 takes over when caspase-9 is absent or can't be activated by Apaf-1.

Ekert and colleagues used growth factor–dependent cell lines...