Most mammalian cells carry two sets of chromosomes, but a few types of cells—including the mainstays of the liver and heart—have additional chromosome sets, a condition known as polyploidy. The advantage of polyploidy for mammalian cells has been unclear. But by tinkering with cells' chromosome endowments, researchers are testing polyploidy's possible advantages. Polyploidy might aid some cells by boosting their size. And in the liver, polyploidy might spur regeneration by creating genetic diversity among cells. Researchers are also working to test whether inducing polyploidy in certain cells can combat a deadly type of leukemia.