Apoptotic DNA fragmentation, the hallmark of apoptosis, is mediated primarily by caspase-activated DFF40 (CAD) nuclease. DFF40 exists as a heterodimer with DFF45 (ICAD), which is a specific chaperone and inhibitor of DFF40 under normal conditions. To understand the mechanism through which the DFF40/DFF45 system is regulated, we analyzed the structural and biochemical properties of apoptotic DNA fragmentation mediated by DFF40/DFF45. Using limited proteolysis, we show that residues 1-281 of DFF45 form a rigid, crystallized domain, whereas the loop formed by residues 277-281 is accessible by trypsin. These results show that the C-terminal helix formed by residues 281-300 is dynamic and necessary for the chaperone activity of DFF45, but not for inhibition of DFF40.