Physiological or programmed cell death plays an important role during a variety of physiological and pathological processes (1 ). It is an active form of cell death under tight regulation. Physiological cell death in mammals occurs during embryonic development of organs and tissues; for example, the loss of excess tissue between digits and the removal of excess neurons during development of the nervous system. Physiological cell death also maintains homeostasis of cell numbers during cyclic remodeling of many adult tissues including the intestinal epithelium, the uterine endometrium, and ovarian follicles, and is critical for the regulation of the immune response. Aberrant amplification of cell death occurs in AIDS and some neurodegenerative disorders. Conversely, cancers and autoimmune disease may arise when cells inappropriately fail to die or undergo mutations that inhibit normal cell death pathways (2 ).
