MAYO CLINIC

Cats come in all sorts of colors—tabby, orange, and calico—but scientists are most interested in the fluorescent variety. Eric Poeschla, a molecular virologist at the Mayo Clinic, used a technique called transgenesis to bestow kittens with a heightened immune system as well as an unearthly green glow, which allows researchers to track the expression of genes of interest.

Poeschla and colleagues used a modified virus to transfer the green fluorescent protein gene into cat egg cells. The green marker allowed them to visualize their real target: a monkey-derived protein called TRIMCyp. This protective protein, found in some primates but not in cats, prevents organisms from becoming infected with an HIV-like disease called feline immunodeficiency virus (FIV).

The transduced cat eggs were then fertilized and injected into adult female cats. Of the 11 successfully-implanted embryos, 10 contained the TRIMCyp...