The present invention relates to methods of determining cancer cell sensitivity to treatment by using antibodies that specifically bind to and detect the presence of a Bcl-2 heterodimer protein, or a caspase and an inhibitor of caspase-IAP heterodimer protein in the cell. The methods provide a predictive tool to identify patients likely to respond to drugs that perturb heterodimer binding and induce apoptosis in a cancer cell. The invention also provides a method of predicting therapeutic efficacy in a cancer patient.