One proven method for tracking down the genetic causes of diseases is to knock out a single gene in animals and study the consequences this has for the organism. The problem is that for many diseases, the pathology is determined by multiple genes. This makes it extremely difficult for scientists to determine the extent to which any one of the genes is involved in the disease. To do this, they would have to perform many animal experiments—one for each desired gene modification.
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Source: Phys.org