You've gotten a couple of answers, but my answer might rely heavily on my field. I work in early stage pharma discovery research. So our goal isn't to determine the basal level of some cytokine, for example, in normal mice. Our goal is typically to see that cytokine's response to treatment, or stimulus, or stimulus then treatment. In other words, the mouse is a living system one step more complicated than a dish of cells, which is its value to us. Sometimes, once you take a drug to non-human primates or humans, you have to drastically change the type of study you run to determine efficacy.