How does a virus make genes fluorescent?
The virus is the transport vehicle for a gene that codes for a protein that fluoresces. If you add this gene to the DNA in an egg cell, induce cell division, implant the blastomere or embryo into a host uterus. The test for success is when the baby kitten arrives - you shine a UV light on it and if you see florescence your attempt at modifying the creature worked. The skin cells are producing the gene's protein.
I think I got that right - close enough.