By further exploration of ABT-737-induced OMP at the cell-free mitochondrial level, we found that (1) cancer cell mitochondria from different sources differed in their sensitivity to ABT-737 correlating with different patterns of (outer) membrane-associated Bcl-2 family members and their interactions, (2) ABT-737 induces Bax, Bak, and Bim desequestration from Bcl-xL and Bcl-2, but not from Bcl-w or Mcl-1. This evidence concerns the gene BAK1 and cancer.