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In the vertebrate cells, ectopic CAD activation triggered caspase activation and subsequent hallmarks of caspase-dependent apoptotic changes, including phosphatidylserine exposure and nuclear fragmentation. Our studies demonstrate that rapid ICAD depletion is sufficient to activate CAD and induce cell death in DT40 and yeast cells. Cell death signals are transduced by death domain. Here we show that caspase 3 cleaves ICAD and inactivates its CAD-inhibitory effect. We have assessed the contribution of apoptosis-inducing factor (AIF) and inhibitor of caspase-activated DNase (ICAD) to the nuclear morphology and DNA. In the accompanying Article, we have identified and molecularly cloned a caspase-activated deoxyribonuclease (CAD) and its inhibitor (ICAD). The Bcl-2 family proteins constitute an important control mechanism in the regulation of apoptosis. Apoptosis is related to many diseases and induced by a family of cell death receptors and their ligands. Apoptosis is also accompanied by the internucleosomal degradation of chromosomal DNA. Taking advantage of the auxin-inducible degron (AID) system, we have developed a suicide system with which ICAD is rapidly degraded in living cells in response to the plant hormone auxin. The ICAD cleavage is consis-tent with DNA degradation, which is a marker for apoptotic cell death16. In particular, it has been unclear whether direct CAD activation in non-apoptotic living cells can trigger cell death. Other aspects of CAD regulation are poorly understood. CAD is normally activated in apoptosis as a result of caspase cleavage of its inhibitory chaperone ICAD. Caspase-activated DNase (CAD) is a major apoptotic nuclease, responsible for DNA fragmentation and chromatin condensation during apoptosis.