In order to achieve this, the team has to inject a virus that holds the DNA of an opsin, which is a protein that can be in light-sensitive cells of the retina, right into the nerves on the paws of lab mice. It took a few weeks before the intended effect kicked in, where only nerves that were involved in pain had the opsin genes present in their DNA. Since the mice lived in a cage that had a transparent Plexiglas bottom, the researchers managed to shine light directly on their paws, which regulated the amount (much or little) of pain that the mice felt. I guess if this were to translate to human use (if at all safe and possible in the future), then it would help medical personnel work on pain management in a more effective manner.