Once we have identified a treatment that seems to work in test tubes and petri dishes, it is time to see whether it works in living things. The most common animal used in these types of experiments is the mouse. We would use a mouse that has been specifically genetically engineered to have a missing or extra copy of the gene that we were evaluating in the lab. These mice would have similar medical problems to the ones seen in individuals missing that particular gene. The mice experiments will tell us many things.
First, we want to know whether this treatment is effective in a living organism. Experiments in a test tube can be rigidly controlled. We can control which reactions go on inside the test tube. We can also control the amount of different substances and proteins we put into the experiment. However, a living organism is much more complex, and no test tube experiment can fully replicate the full range of reactions and variables that are present in an animal. We want to know whether, in the complex environment of a living organism, the drug either reduce or enhance gene expression.
Another goal of these experiments will be to see whether the drug actually makes the problems better. Does compensation for the extra or missing gene actually cure the problem, or, at the very least, improve it? For example, if we know that a gene is linked to dysmyelination, does treatment with this drug candidate improve myelination in these mice?
Third, we want to get an idea about the potential side effects of this drug in living organisms. If the drug successfully treats the underlying problem but the side effects are worse than the original problem, it is probably not a good drug candidate.
Once we have determined that a particular drug is effective and safe in mice, we can move on to the next stage: clinical trials!
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