Apoptosis has come to be used synonymously with the phrase “programmed cell death,” as it is a cell-intrinsic mechanism for suicide regulated by a variety of cellular signaling pathways (for a recent review, see ref. 2). The apoptotic machinery, initially identified in Caenorhabditis elegans, is evolutionarily conserved in higher organisms, and apoptosis represents a default mechanism in mammalian development. It is marked by a well-defined sequence of morphological changes. For neuronal death to be classified as apoptotic, nuclear condensation and fragmentation, cleavage of chromosomal DNA into internucleosomal fragments, and packaging of the deceased cell into apoptotic bodies without plasma membrane breakdown must be observed (Fig. 1).