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. The first few test subjects had tumors that were deep in the brain, so the surgeons had to remove a piece of the tissue before shining a light on it. "[T]he question was, 'Does it glow?' And when we saw that it glows, it was just one of those moments...'Wow, this works,'" Chirag Patil, one of the researchers behind the test at Cedars Sinai Medical Center in Los Angeles, told NPR.