Role of transgenic animals in drug development

First, one can test whether it is reasonable to pursue a certain therapeutic approach. For example, if the researchers want to know if a disease could be treated by reducing the amount of a certain protein or the activity of a particular enzyme, they can produce transgenic animals missing the corresponding gene ("gene-knockout animals")
If symptoms of the disease are ameliorated without serious side effects, then it might be worth the time and expense of producing a drug that would have the same effect