CPS-6 normally controls a process of programmed cell death that helps organisms keep old cells and new cells in balance. But Xue’s team found that during fertilization, CPS-6 could also move into the innermost part of the mitochondria and chop the mitochondrial DNA stored there into pieces. That DNA spells out instructions for critical tasks carried out by the mitochondria. Without the instructions, the mitochondria can’t do their job.