Jellyfish DNA was inserted into the cloned rabbits, making them glow. It doesn’t hurt the bunnies at all, it is just visual confirmation that the insertion was successful. Researchers hope to use this technology to make better drugs for hemophilia by creating blood clotting enzymes more efficiently. The two glowing bunnies are perfectly healthy, and cannot be distinguished from their siblings in the daylight, but under UV light it is hard to mistake who has the gene.
In the past, cats have been made to glow in the dark to show they had an antibody for feline immunodeficiency virus (FIV).