Since caspases have not yet been identified in single-celled eukaryotes (Aravind et al.y 1999), it appears that the caspase-dependent cell-death pathway may have arisen coincident with the evolution of metazoan animals. The process of pro grammed cell death, by comparison, appears to have evolved much earlier and per haps several times independently, since a number of single-cell eukaryotes, including the slime mold Dictyosteliunĩy the ciliate Tetrahymena, and the protozoan Trypanosoma bruceiy exhibit cell-death programs but appear to lack caspases (Davis etai, 1992; Cornillon et al.y 1994; Ameisen etai., 1995). These cell-death programs are accompanied by some, but not all, of the morphologic features characteristic of apoptosis. In the case of Dictyosteliumy cell death has been shown to be associated with release of apoptosis-inducing factor (AIF) from mitochondria. This cell-death program appears to be an alternative to the caspase-dependent pathway. Since it is also present in higher metazoans (Susin et al.y 1999), it may represent an ancient form of programmed cell death which has been maintained in evolution.