Cancer travels. Large tumors shed cells that move through the body and seed new malignancies. Now scientists are tinkering on the nanoscale to build unusual spheres made of DNA—a molecule that became famous as another shape, the double helix—that can find, tag and potentially kill off these tumor cells.

The spheres look a bit like toothpicks stuck in a small Styrofoam ball. The toothpicks are really a dense crowd of single DNA strands jutting out from a central core. The strands are chosen for their ability to bind to complementary DNA in cancer cells. When a bond happens, it displaces tiny light-emitting molecules stuck to the tips of the DNA in the sphere, essentially sending up a flare that indicates the presence of cancer. The brighter the flare, the more cancer DNA that is present, says Chad A. Mirkin, a chemist and director of the International Institute for Nanotechnology at Northwestern University, who has spearheaded the research.

These encounters occur in a sample of a patient's blood. When the spheres run into a cell, they move through pores in the cell membrane into the interior. There, the DNA that forms that outer rim has a much higher chance of encountering and latching on to cancer DNA than isolated strands would.* Spherical nucleic acids “bind to other nucleic acids 100 times more strongly,” Mirkin says.

Mirkin's spheres, also called Nanoflares, are already being used by hospitals for rapid cancer diagnosis. Other systems fish out dead tumor cells based on proteins on their outer surfaces, but because these spheres identify live cells, Mirkin says, scientists could test how the cells respond to different drugs and eventually develop personalized treatments based on the results.

*Editor's Note (6/2/15): This sentence was edited after it was posted online to remove an error that appeared in the original print version.