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For the longest time around, in most research studies on health and medicine, mice have been the favorite "substitute" for humans in experiments or tests. The popular choice has been said to have similarities in human genes that would make the perfect fit for drug testing. The ordinary lab mouse has always been the go-to option for learning more about our own species because of its big population that makes it readily available for tests as well as its being less expensive than other species.

An international consortium study found humans and mice have a lot in common in terms of how our genes are expressed and regulated. According to reports, majority of the mouse's genes that have endured through evolution are "conserved", leaving them with the same functions after all these years, just as humans have.

"This is the first systematic comparison of the mouse and human at the genomic level," said Dr. Bing Ren, a professor of cellular and molecular medicine at the University of California, San Diego, and a co-senior author of the new study, in a statement.

Ren and colleagues from around the world discovered that many of the processes and pathways are shared between humans and mice. "This allows us to study human disease by studying those aspects of mouse biology that reflect human biology," Ren said.

"The genes that are more consistent across mouse tissue than when compared to the corresponding tissues in humans are those involved in metabolism," he said. Since a mouse heart beats 10 times faster than a human's, it's not hard to believe that the two organisms have very different metabolic systems.

In general, the researchers found, the systems that control gene activity are largely the same between mice and men.

"The assumption has long been that whatever was discovered in the mouse would likely be true in humans too, but the idea has never been systematically evaluated and assessed," Ren said in a statement.

"There are a substantial number of mouse genes that are regulated in ways different from similar genes in humans," Ren said. "The differences are not random. They are clustered along certain pathways, such as in genes regulating the immune system."

The study, however, also showed some dissimilarities between mice and human genes. Immune system processes and some related to metabolism and stress response tended to differ in mice.

Dr. Feng Yue, first author of the study and Penn State University microbiology professor, said "We want to say that, indeed, mice are probably the best system to use for studying these two systems. But if you're studying the immune system using mouse models you probably need to be more careful and perform more functional experiments. It's not saying that their findings are wrong."

The study came from a research effort called Mouse ENCODE, an offshoot of the Encyclopedia of DNA Elements (ENCODE) which began in 2003. ENCODE catalogs elements of the human genome that guide cell function. Mouse ENCODE looked at over 100 mouse cell types and tissues, comparing them to their corresponding elements in humans.

"In general, the gene regulation machinery and networks are conserved in mouse and human, but the details differ quite a bit," said co-senior author Dr. Michael Snyder, director of the Stanford Center for Genomics and Personalized Medicine.

"By understanding the differences," Snyder added, "we can understand how and when the mouse model can best be used."

While mice models have been researched time and again for their differences to human models, scientists had yet to understand how they are different or how much they are different. Now they can use these important differences to pinpoint where similarities actually lie.

"By the most comprehensive studies," Yue said, "we confirm it is, indeed, a good system to use."

"[A] lot of the genes are identical between a mouse and a human, but we would argue how they're regulated is quite different," Snyder added in a statement. "We are interested in what makes a mouse a mouse and a human a human."