Programmed cell death(PCD),unlike the other form of cell death(necrosis),is an active process of cellular suicide.It plays an important role in embryogenesis,tumorigenesis and clonal selection in the immune system. bc1-2，a potential physiological regulator of PCD,can not block all kinds of programmed cell death mediated by various stimuli.bc1-X,a recently found gene which encodes two different proteins,takes an important part in both positive and negative regulation of PCD. bc1-2 is regarded as a member of the third category oncogenes because its blocking of PCD results in tumorigenesis.