The human breast cancer cell line MCF-7 transfected with plasmids expressing either Bcl-2 or Bcl-XL is an excellent system in which to examine the differences between these two proteins as the cells do not express detectable Bcl-XL, and endogenous Bcl-2 can be drastically reduced by growth in estrogen depleted medium [23]. Here, BCL2 is linked to breast cancer.