BIRC2 and neoplasm: To clarify whether the loss of cIAP1 or cIAP2 played a dominant role in the tumor microenvironment in reducing the number of tumor nodules, we utilized ciap1−/−ciap2frt/frt (ciap1−/−) and ciap1fl/flciap2−/− (ciap2−/−) mice to mimic the effect of Smac mimetics on the host alone.