Apoptosis can, therefore, be induced by expression of wild-type p53 in undifferentiated leukemic cells. This apoptosis inducing effect of wild-type p53 was confirmed in other cell types (reviewed in refs. 33 and 81). Induction of apoptosis by wild-type p53 in Ml myeloid leukemic cells was inhibited by cytokines such as IL-6 (94) and interferon-g (87). Experiments with p53 knock-out mice have shown that wild-type p53 is also involved in mediating apoptosis in normal myeloid precursors deprived of the cytokines required for cell viability, and that this induction of apoptosis by wild-type p53 in normal myeloid precursors is inhibited by adding the appropriate cytokine concentration (96). Experiments with p53 knock-out mice have also shown wild-type p53 mediated apoptosis after irradiation in myeloid precursors (96), thymocytes (96, 98, 99), and intestinal epithelium cells (100) and that there are wild-type p53 dependent and p53 independent pathways of inducing apoptosis (96, 98, 99).