The regulation of apoptosis is an essential mechanism for physiological cell death and tumorigenesis. The analysis of the genetic machanisms reponsible for susceptibility to or protection from apoptosis stimuli not only enables us to gaininsight into tumor cell biology,but may also prove useful in the development of new therapeutic strategies. Among the proteins coded by the genes of BCL2 family,BCL2,BCL-X_L,MCL-1,CED-9,BAG-1,E1B-194 and A1 act as death repressors, whereas BAX, BAk, BCL-X_s and BAD promote cell death. These different functions were shown to be mediated by homodimerzation/heterodimerization of the various family members. Therefore a critical balance between molecules above may determine the fate of cells in reponse to apoptosis-inducing conditions.