In the developing chick nervous system, large numbers (-50%) of sensory and spinal motor neurons undergo apoptosis (2,12). In the rodent nervous system, apoptosis occurs as well (Figs. 1 and 2). Studies on developmental cell death need to be done in primates. Apoptosis occurs continuously in cell populations that undergo slow proliferation (e.g., liver and adrenal gland) or rapid proliferation (e.g., epithelium of intestinal crypts) (13,14), and it occurs as a normal event in the deletion of lymphocyte clones after an immune response (15).