Brain surgery is notoriously complicated. Before surgeons go in to remove a tumor, they study the size and location of the tumor. But once they're in, they have to rely on their fingers and eyes to distinguish tumor cells from healthy brain cells. Now researchers have developed a "paint" that can be injected into a patient's veins to make tumor cells glow. After a number of successful studies in mice and dogs, the paint is now being tested in humans in California.

The paint is made from two chemicals. The first is chlorotoxin, a protein derived from scorpion venom, which targets the chloride receptors on the surface of tumor cells. The protein carries a second, non-toxic chemical that makes the cells fluoresce when they are exposed to near-infrared light.

Tests at this early stage are designed to make sure that the paint works as it's supposed to, and initial results have been promising. Researchers injected the tumor paint into the patients' veins and it was successfully able to cross the blood-brain barrier, which protects the brain from harmful chemicals that might be in the blood.