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Here’s the how: last year, a team of researchers led by Alejo Menchaca at the Animal Reproduction Institue of Uruguay genetically modified some wooly ruminants to express a peptide commonly known as green fluorescent protein, or “GFP.” GFP glows green when exposed to ultraviolet light, and was originally isolated from the jellyfish Aequorea victoria. Today, it’s used to label molecules of biomedical interest – to determine whether those molecules are being expressed, and in what quantity. In the last twenty years, GFP and its derivatives have become such ubiquitous and invaluable research tools that its discoverers were awarded a Nobel Prize back in 2008.

Menchaca’s sheep – which were born last October and, apart from their corporeal glow, appear to have developed just like an unmodified flock – express the protein in tissues throughout their bodies, causing them to glow under ultraviolet light. Menchaca says that, in this particular instance, the sheep were modified merely as a proof of concept. As in: Look everybody, we can breed a totally healthy sheep that also happens to glow. Isn’t that swell? Well yes, it is. It’s kind of eerie (look into the eyes of the GFP-sheep up top and tell me you aren’t afraid), but also pretty cool. But why make a sheep glow in the first place?

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Why?