Why Do Scientists Use Mice? The Hidden Heroes of Medical Research
Hidden Heroes Behind Medical Breakthroughs
Every major medical breakthrough has a quiet cast of characters who never get thanked in the headlines. They don't wear lab coats, don't hold pipettes, and don't get their names on a single research paper — but without them, most of modern medicine simply wouldn't exist. They're mice. And their story is worth telling on its own.
Little but Mighty Mice
Mice make up the overwhelming majority of animals used in biomedical research, and for good reason. They share roughly 85% of their protein-coding genes with humans, breed quickly enough that researchers can study several generations in a short time, and are small and inexpensive enough to house in large numbers. But their real power lies in something more specific: mice can be genetically engineered with a precision that's simply not possible in humans — letting scientists switch a single gene on or off and watch, in real time, what that gene actually does inside a living body.
Genetically Engineered Mice
Much of what we know about how diseases like cancer develop comes from transgenic mice — animals genetically modified so that a specific gene is switched on (or off) in every cell of their body. This lets researchers isolate the effect of a single gene, something almost impossible to study directly in a human patient.
One well-known example involves cancer-driving genes called oncogenes. When researchers engineer mice to switch on an oncogene, the mice don't always develop cancer immediately — often, they pass through a slow, gradual disease process strikingly similar to how cancer actually develops in humans. That slow progression turned out to be scientifically valuable in its own right: it revealed that switching on a single growth-driving gene isn't enough to make a cell cancerous. The cell also has to switch off its own self-destruct mechanism — a built-in safety system that would otherwise stop it from growing out of control. Understanding that second step became central to an entire generation of modern cancer drugs, which work precisely by forcing that self-destruct switch back on.
The Eμ-Myc transgenic mouse model overexpresses the myc oncogene in B lymphocytes, making it a highly reliable pre-clinical model for studying aggressive B-cell lymphoma. Because these Myc-driven lymphomas survive by relying heavily on anti-apoptotic proteins, the model served as a foundational tool for testing the efficacy of the selective BCL-2 inhibitor venetoclax.
Humanised Mice
Ordinary transgenic mice modify one or two genes at a time. Humanised mice take the concept a step further: instead of tweaking a mouse's own genome, researchers strip away the animal's immune system entirely and rebuild it using human cells — human stem cells, human immune cells, sometimes even fragments of human tissue.
The result is a mouse whose immune system is, functionally, a human one. This matters because mouse and human immune systems, while similar, don't always behave the same way — a drug that looks promising in an ordinary mouse can behave completely differently once it meets real human immune cells. Humanised mice close that gap. They've become essential tools for testing cancer immunotherapies, studying how infections spread through human tissue, and modelling autoimmune diseases that don't occur naturally in mice at all.
Humanized mice are vital for Long COVID research because standard laboratory mice are naturally resistant to human SARS-CoV-2 and lack human-like immune systems. By using models with humanized immune systems or tissues, scientists can effectively replicate the complex chronic inflammation, multi-organ damage, and autoimmune features seen in human patients.
It's a strange kind of chimera — a mouse carrying someone else's immune system — but it may be the closest thing researchers have to safely testing a new therapy on a human being before any human is actually involved.
Animal Ethics
None of this happens without a serious ethical conversation attached to it. The mice used in medical research don't consent, and some of them get sick or are euthanised so that, one day, humans might not have to face the same disease. That tension doesn't disappear just because the research is important — it gets managed, through a framework researchers around the world refer to as "the 3Rs."
Replacement asks whether an experiment could be done without an animal at all — using cell cultures, computer modelling, or human tissue samples instead. For many basic biological questions, no substitute yet exists for a whole, living, breathing organism.
Reduction asks whether the fewest possible animals were used to get a statistically reliable answer — no more, no fewer. Careful study design, planned before an experiment even begins, has dramatically cut the number of animals needed compared to a few decades ago.
Refinement asks whether animal suffering was minimised at every step — through pain relief, humane housing, and ending a study the moment enough data has been collected.
In most countries with active biomedical research sectors, these principles aren't just good practice — they're law. Any institution using animals in research must have its studies approved in advance by an independent animal ethics committee, and no experiment proceeds without that sign-off.
None of this makes the ethical question disappear. It's a genuine trade-off, weighed case by case, between the suffering of a small number of animals now and the possibility of preventing suffering in millions of people later. The mice behind decades of medical progress didn't get a say in that trade-off. The researchers who work with them do — and the modern framework of animal ethics exists to make sure that responsibility is taken seriously, every single time.