That naturally occurring cell death in the nervous and other systems is an active and physiologically appropriate process has received much attention recently and has gained a significant degree of acceptance. The identification of cell death genes in invertebrates, the characterization of gene products that function as cell death suppressors, and the demonstration that some proto-oncogenes elicit cell death, as well as proliferation, in certain cell types have heightened interest in the mechanism of programmed cell death. Yet, evidence for a genetic program for cell death in vertebrates remains circumstantial and, so far, vertebrate 'cell death' genes exist only in theory.