Roughly 95% of all lab animals are mice and rats. I have long wondered why mice and rats were used in such high numbers in animal testing considering how seemingly different they are to humans. These rodents look nothing like us for a start, so why use them as a frame for determining how humans will react to drugs, social interaction, environmental changes and food additives, among many other things? There are many reasons for this, because the rodents are small and easily housed, fast breeding, have a short lift expectancy (2-3 years) and are inexpensive compared to other animals. However, there are still further reasons other than convenience. We actually share many genetic similarities. Estimates of the number of genes in the human genome have varied wildly in recents times, from around 2,000,000 in the 1960s to the more modest figure of between 20,000 and 25,000 at the time of this article. This figure is significant because mice have approximately the same number of genes in their genome also. From this point onwards I will be focusing on mice as there is more information available about them. On average, the genes within both the mice and human genomes are 75-85% identical, with some genes being 99% identical and others around 60-70% identical.
The mouse was the second mammal (after the human) to have its genome sequenced. At the present day over 20 mammals have had their genome sequenced as well. This collection of genome sequences allows for comparisons of genes between species and provides clues for evolutionary pathways (something I desperately need to learn more about).
Anyway, back to medical research, apparently 90% of genes associated with disease are identical in humans and mice. This means that mice which have a particular susceptibility to certain diseases can be observed and tested in order to provide quite accurate prediction of how a human body or brain would react to the same disease.