Viability is a fundamental determinant of all cellular functions, and regulation of cell viability is important in shaping tissue differentiation and organism development. The genes in the bcl-2 family can profoundly influence cell viability. In addition to bcl-2, cellular genes in this family include bcl-x, bax, mcl-1, and A1 (from vertebrates), as well as ced9 (from Caenorhabditis elegans). Different members of the family exhibit a spectrum of activity, ranging from survival-enhancement of death-enhancement. Thus, the combinatorial effects of various bcl-2 family members may allow a fine level of control over the important function of cell viability. In addition, alterations in these genes may cause aberations in cell death and thus contribute to cancer.