Pigs that glow from inside out and glow-in-the-dark cats and dogs may sound, and look, pretty ridiculous, but scientists are increasingly using genetically modified animals in an effort to help them understand diseases that affect humans. Or sometimes just for art.

One way they are doing this is by inserting fluorescent proteins, generally a green protein found in the Aequorea Victoria jellyfish, into animals, making them glow. The fluorescent proteins help scientists to monitor the performance of genes that they have altered. The pioneers of this method were even awarded the 2008 Nobel prize in chemistry for their efforts. Below are seven genetically modified animals that now glow thanks to science:

1. Sheep

In 2012 scientists at the Animal Reproductive Institute of Uruguay injected newborn sheep with the green fluorescent protein found in the jellyfish. As the sheep grew, they gave off a greenish glow under certain ultraviolet light. According to the scientists responsible, the sheep are healthy and behave normally. The lead researcher, Alejo Menchaca, said the experiment was done help refine the technique.

2. Pigs

Image credit: Metafilter

We have had partially fluorescent pigs before, apparently, but in 2006 scientists at the National Taiwan University claimed to have successfully bred three green pigs using DNA from the fluorescent jellyfish that was added to pig embryos. The result was three pigs that glowed green inside out, including their internal organs. Scientists said the pigs would be used in stem cell research and the study of human diseases.

3. Monkeys

Image credit: Cherry Bombed

In 2009, Japanese scientists, gave the same jellyfish DNA to monkeys, which was then inherited by their young, the first time a genetically modified animal had passed on such genes. Researchers said that they planned to breed families of monkey that develop neuro-degenerative diseases in order use them to better understand Parkinson's and motor neurone diseases.

4. Dogs

Image credit: Singularity Hub

In 2009, scientists at Seoul National University produced the world's first transgenic dogs by cloning fibrolast cells that produces a red fluorescent protein. The five dogs glow in the dark or under ultraviolet light and all produced glowing offspring. In 2011, a group of researchers at the same university then bred a dog in which the glowing effect could be controlled, effectively turned on and off. Scientists said the study will help them to understand "genes that trigger fatal human diseases," especially given that humans and dogs apparently have 268 illnesses in common.

5. Cats

Image credit: Mayo Clinic

In 2011, researchers from the U.S. and Japan used the green protein to help them monitor the activity of a gene they had inserted into cats which helps them resist the feline form of Aids. Cats are one of the "few animal species that are normally susceptible to such viruses" and scientists say that the experiment is designed to help better understand Aids in both cats and humans.

6. Fish

Image credit: University of Exeter

A team of scientists genetically engineered zebrafish and used the green protein to help track what industrial pollutants, specifically the endocrine disruptors found in them, do inside the body of the fish. The idea of the 2012 study was to gain a better understanding of where endocrine disruptors may be harming the body, research that will help understand the health impacts of pollutants in humans.

7. Rabbit

Image credit: Chrystelle Fontaine

In 2000, artist Eduard Kac, known for using genetic engineering to create living works of art, convinced a French research institute to breed a rabbit that glowed green in blue light to be part of a project designed to provoke debate around the practice of genetic engineering in animals. The rabbit died two years later and scientists labeled the controversial project "frivolous."

The above animals certainly make for a weird looking bunch. Woodland Hastings, a biologist at Harvard University who helped discover the jellyfish gene and its function, says that as far as scientists know, there is "nothing dangerous" about the technique itself. So as these types of studies increase, are the above animals weirdly awesome creations that are advancing scientific understanding of human diseases, or a weirdly cruel and unnecessary use of animals?

What do you think? Let me know in the comments below.