From Science:
Those smelly mothballs grandma has in her closet do more than just keep her sweaters free of holes. They cause cancer in mice and possibly humans, and now researchers may have figured out why. A study of the nematode worm Caenorhabditis elegans suggests that chemicals in mothballs shut down the natural process by which cells commit suicide, allowing cancer cells to divide and conquer.
The finding is serendipitous. When a neighboring lab became infested by mites, biochemist Ding Xue of the University of Colorado in Boulder tried to protect his C. elegans by putting mothballs in their containers. But the balls may have done more harm than good. Some of the worms’ cells that were programmed to apoptose–or kill themselves–kept living indefinitely, a problem that has been linked to cancer in humans.
More here.