Are there any problems where several genes need to be replaced?

We put this question to Tim Cox from Addenbrooke's Hospital...

Well, it's a very good point that the conditions, what we refer to as the single gene defects, are often not so simple, and that they have modifying factors and other genes that contribute to the development of what we refer to as the disease phenotype, the condition that you see in the patient. But, on the whole, there's a primary gene that we've chosen that's defective and which then drives the development of the disease. And when you put that right, one hopes very much that the other thing will fade into insignificance. So that's why, by starting in this area, we choose the definitive approach to major determinants that are predisposed to disease. That is, choose your weapons, and choose your targets first, I think. But it's quite true that, once we begin to understand diseases that have more complex genetic architecture, it may be necessary to tackle those and to think about a multi-pronged approach, focused in each case to the determining locus. But I think that's a bit far ahead. Let us deal with what we've got in our clinics at the moment before we start tackling the really complex things.