Mice are used in a wide variety of scientific projects. Mice breed very quickly, so scientists often use mice to study questions about genetics. For example,they can breed mice with a particular disease and see how many of their offspring also have that disease. This may tell scientists whether the gene for that particular disease is dominant or recessive. Even though mice look very different from humans, both mice and humans are mammals, so many aspects of our biology are pretty similar. As a result, scientists can use mice to test whether drugs designed to treat diseases will be effective and non-toxic to humans.

This question is worthy of a book, really. Mostly, medical researchers use mice as a replacement for humans in their studies. For example, scientists are trying to develop a cure for diabetes using stem cells. They take stem cells, which are cells that can become virtually any cell in your body, and expose the cells to certain chemicals that cause them to develop into cells similar to islet cells. Islet cells are in our pancreas and make insulin. Diabetics do not have functioning islet cells and so need to take insulin injections to control their blood sugar. Once the researchers made the islet cells in the lab, they needed to inject them into a living organism to see (a) if the cells will make enough insulin to cure a diabetic and (b) if the cells will have any harmful effects. Because it is not ethical to test out new drugs on humans, we test them out on animals that have similar immune systems and metabolisms. Mice are one such animal. They are small, so they are easy to keep in a lab, and they are similar to humans in their immune systems and many organ systems, like the pancreas.

Another study I am aware of that researchers used mice in was a study of cystic fibrosis. Cystic fibrosis is a genetic disease that is very, very harmful. People who have it survive, but only with lots of medical care, and even then they don't usually live past 40 and they can't have kids. Biologists wondered why the disease is so common in white people. The genetic mutation that causes cystic fibrosis is thought to have arisen about 50,000 years ago in Europe. That's plenty of time for natural selection to make the allele very rare in the human population (we didn't have hospitals 50,000 years ago so most people with the disease would have died). And yet the disease is pretty common still today. So researchers thought that maybe having one copy of the allele might increase survival somehow. So they used mice to experiment. They inserted the mutation that causes cystic fibrosis into mice, and exposed the mice to different diseases. They found that mice with two copies of the mutation died but mice with one copy of the mutation were more likely to survive diseases that cause severe diarrhea. This study suggests that the cystic fibrosis allele has both a harmful effect on the human population (if people have two copies, they are likely to die from cystic fibrosis) and a beneficial effect (if people have one copy they are less likely to die of diarrhea).

In the past, before good medical care, most people died from diarrhea-causing diseases like typhoid and cholera, so having one copy of the mutation would have increased your chances of survival, which would have helped you to have more children, making the allele more common in the population. This is a very interesting experiment to those of us that are curious about whether humans are still evolving, and a potentially important study to people trying to prevent children dying from diarrhea, but it is certainly not an experiment that could have been done on humans.

Mice have been used in many projects including Cancer Research, Behavioral experiments, dieting experiments, and various Genetics experiments. As an example of one experiment in genetics. Scientists bred mice with certain coat colors and determined what hair colors and patterns were dominant to others. They also found that there are several genes involved in hair color and patterning and even that a certain combination of the genes is lethal.