Really. Scientists have created rodents that glow like fireflies.
Their experiment involved first freeze-drying mouse sperm, then inserting DNA from another species into it and finally injecting the altered sperm into mouse eggs. The gene inserted in this case was from a jellyfish and makes the sea creature glow fluorescent green. It was used because it was easy to see if the gene had been successfully incorporated in the mice - the mice glow too.
This isn't a parlor trick. Scientists believe this ability will allow them to pinpoint the effect of new drugs better, especially in the treatment of cancer. Picture glowing cancer cells dim and then disappear as an anti-cancer drug takes effect.
By the way, the glow can be inherited. So, if you see a green object scurry across the room, you'll know there's been an escape from the local lab.
(Mickey, "Minnie, you must be happy, you're positively glowing!" )